Hey, Berries! I saw that Behind the Name has the history of nicknames, so I wanted Nameberry to have one too!
Nicknames started with the phrase “an eke name”, with eke meaning also. Over time, it evolved into “a nekename”, which was translated as “nickname” to English speakers.
Some nicknames definitely puzzle some people, like how you get Billy from William and Peggy from Margaret (although you Berries are knowledgeable 
). Hopefully this will help!
“Obvious” Nicknames
Some of the most common nicknames, especially in the English language, stem from either the first, middle, last syllable of the name. These names would, of course, be Lil from Lillian or Beth from Elizabeth. A majority of these shortened forms were also given an “ie” or “y” suffix, ending in Abby (Abigail) or Johnny (John). These names were common for both children and adults to say, as it was also a way to differentiate one person from another.
Examples:
Lilly / Lillie / Lily from Lillian
Gwen from Gwendolyn
Bea from Beatrice
Maddie from Madeline
Sam from Samuel
Johnny from John
Georgie from George
Rhyming Nicknames
Beginning in Medieval times, rhyming nicknames were a fun way children referred to each other. While William may have been primarily called Will, his friends might have rhymed and called him Bill. Just the same, a Margaret called Meg may have had her nickname rhymed to Peg, which then evolved into Peggy.
Examples
Margaret = Meg → Peg → Peggy
Rose = Rosie → Posie
Josephine = Josie → Posie
Robert = Rob → Bob → Bobby
William = Will → Bill → Billy
Cultural Nicknames
Whereas the above were common English practices, other cultures are known for their variety of nicknames.
The Russian culture tends to add many suffixes, among them being “-ya” and “-sha”. Among these would be Sasha from Aleksandr(a) and Anya from Anastasia. Some other suffixes used in that language would also be -ik, -nka, and -shka, according to Behind the Name.
The French add “-ette” and “-ine” to many feminine names— among others— to either turn a masculine name feminine or a sweet nickname. Examples would be Jean → Jeanette or Jeanine; Paul → Paulette or Pauline; and Georges → Georgine or Georgette. There are also nicknames that do not follow this pattern and are French: Coco, Fifi, Gigi, and Kiki. These are simply repeats of a syllable, such as Coco from Cosette, Gigi from Giselle, and so on.
Many Italian nicknames end in “ina”, “ino”, “etto”, and “etta”. Like the French, it can make a masculine name feminine. These suffixes are found in Luigina (from Luigi), Giuseppino (Giuseppe), and Marietta (Maria).
The Spanish language primarily includes nicknames ending in “ito” or “ita”, while other nicknames such as Isa (from Isabel) don’t necessarily follow this format, and are borrowed from the first syllable.
This is also an important thing to note:
A bit of insight from @choupette! ^
The Dutch sometimes use the suffixes “-je” or “-tje”, whereas many Eastern European languages use the Russian suffixes (see above)-- or “-ska” (Hungarian), “-uše” (Czech), and
“-ka”/“-ko”.
While some of these are commonly used as first names (Jeanette, Georgine, etc.), they historically evolved as nicknames, just as Lily is a stand-alone name that can also be short for Lilian or Liliana.
And here, a special feature on Croatian nicknames written by @/krijesnica
Croatian Nicknames
My country, Croatia, has a long tradition of nickname names as full names, as well as using nicknames on the regular. If your name is Magdalena, there’s an extremely high chance you will be called Megi (prn. like Maggie) almost all of the time. Dorotea will almost always go by Dora, and Antonela by Nela.
Some of these nicknames/diminutives have become names in their own right and are used as such, like Tin (short for Martin, Valentin, Augustin), Toni/Tonči/Tonći (short for Antonio), Nika (short for Nikolina), Pero (short for Petar). If someone told you their name was Nika you wouldn’t assume it’s short for anything, you’d assume it’s their full name.
What you wrote about Russian diminutives and suffixes also goes for us, albeit with different suffixes! Most popular suffixes include:
- -ka (Anka, Ivka) - feminine
- -ica (Anica (f), Marica (f), Nikica (m), Perica (m)) - this suffix is used for nicknames of both sexes
- -ša (prn. sha; Lukša, Jakša) - masculine
- -ko (Joško, Paško) - masculine
All of the names I listed are currently used as full names, and you are just as likely to meet a Josip who goes by Joško as well as someone whose full name was Joško.
We are big on nicknames in Croatia, and if you have a shorter name (3-4 letters), which has been popular in Croatia for the past 20 years, or if your name is already a diminutive, there’s a high chance your nickname will be longer than your name! Of course, not everyone has a nickname, but it’s hard to find someone who doesn’t at least have some sort of a family nickname.
And an invaluable bit of Swedish nickname knowledge from @Foulbaubleofmansvani
Swedish Nicknames
Step 1: Pick a syllable. If the original name has one, you pick everything before and including the (first) vowel. If the name has two syllables, you pick the first. If the name has more than that, you have some creative liberty.
Step 2: Pick a consonant from whatever is left of the name. Double it. If you picked K, do CK instead of KK.
Step 3: Add -E or -An to the end. E is mostly for boys and An is mostly for girls but there is some overlap.
Examples:
Karl → Ka → Kall → Kalle
Lars → La → Lass → Lasse
Ludvig → Lu → Ludd → Ludde
Marie → Ma → Marr → Marre
Margareta → Ma → Magg → Maggan
Birgitta → Gi → Gitt → Gittan
Sometimes one name has several nicknames this way. Sebastian can be Sebbe or Basse. Sometimes several people in the same class will have the same nickname (and the ones that will actually get called by it will probably be determined by popularity). For example, Marie, Maria, Martin and Marcus could all be Marre/Marran. Marcus could also fight Max about who should be Macke/Mackan.
Possibly Confusing Nicknames (English)
Many people, primarily children, found it hard to say names with a “th” or “r” sound. This is how certain nicknames arose such as Molly or Polly for Mary; the “r” sound was changed to “L”, making it Molly, then using the rhyme to achieve “Polly”. This is also shown in Buffy for Elizabeth (Beth → Bethy → “th” to “f” → Buffy), and Sadie / Sally for Sara(h).
With some names, such as Charlotte, the pronunciation was altered slightly, making a nickname such as Charlie pronounced “CHAAR-lee” instead of “SHAR-lee”, from Charles.
According to Behind the Name, the word mine was commonly used in front of names by relatives. When it was said quickly, mine Anne was turned into my Nan, resulting in the older nicknames of Nan and elaborated Nancy for Ann(e) or Anna. If you are ever confused on how a name like Nell is a nickname for Eleanor, try saying the phrase “mine El” quickly! 
Some nicknames are also borrowed from other languages; have you ever met a Margaret called Daisy? well, that all started because Margaret in French became Marguerite, the coincidental French word for daisy. So Marguerite was affectionately called Daisy, which turned into Margaret being called the same!
Cute Nicknames
Inspired to add this from the Cutest Celebrity Baby Names post by @sophiekihm!
Cute nicknames are used universally, as most nicknames are considered cute. However, there are some that are cuter than others— a lot of it comes from preference! There are a few different categories when it comes to nicknames; they are~
Repetitive Sounds
Some of the very most French-inspired nicknames are Fifi, Coco, Gigi, Kiki and company. These names all draw from the repetition of sounds featured in the names. These repetitive nicknames have given inspiration to nicknames such as Didi, Lulu, or Jojo to come from names with those prominent sounds, sounding bubbly and cutesy by the end of it.
Nicole → Coco
Gianna → Gigi (GG)
Josephine / Sofia → Fifi
Cecilia → Cece (CC)
Pet Names
Some of the cutest nicknames may be used at home only. After all, you wouldn’t put “sweetie-pie” on a school exam because that’s what your mum calls you. Examples of these names would be Sweetie, Honey, Doll, Baby, among others. (Although Honey could make a cute name <3)
“EE” Endings
The next kind would be names that are most definitely appropriate to introduce yourself as! Some of the cutest names end in the “ee” sound, and make a longer, sometimes “tougher” name sound sweet yet not too sugary. This also is used with a “zee / see” sound, such as calling your May “Maisie” or Olivia “Livsie”.
Such as…
Ellie from those names with a prominent “El” → Elizabeth, Ella, Eleanor, Eliana, et al.
Maisie from those “May” names like Mary and her cousins!
Allie from the “Al” names
And with that said, here is the most used nicknames as first names, updated as of 11/27 with data from the US 2022 Name Charts
The Most Used Nicknames as First Names
To finish this little history, I analyzed the SSA Top 100 for which “nickname names” are used the most as standalones in the US (2022).
GIRLS
Mia still stands at the most used nickname-name for girls, with 0.62% of females born carrying this name, which originated as a nickname for Maria and is now used 0.4588% more than her parent name.
Mila surpassed her spot of #26 from last year, coming in at #19 as the lilting diminutive of Camila, Milena, et al. now used on her own. Still trailing Camila, where 0.4464% of girls bore that name, Mila comes with 0.3612% of females bearing the name on its own, with a less than 0.01 difference in percentage of girls born. Mila outranks Emilia by 0.0998%, as well as Milena, which is not ranked in the top 100.
Gianna, used as a nickname for Giovanna in her home country, is the name of 0.3578% of girls born in 2023, lowering in popularity from last year but still ranking high at #22.
Nora sometimes used as a short form of Honora, (E)leonora, and other names ending in that sound, Nora was the vintage choice of 0.3429% of parents, whereas Eleanor was the popular choice by -0.001877%, outranking Nora by 12 places.
Lily rose in rank to move past Ellie with 0.3343% of baby girls named Lily, a nickname for all the “Lil” names. She outranks them all, beating out Lillian by -0.001956%, and Liliana by -0.000716%, meaning they are the only “Lily” names that rank in the top 100.
Ellie, the popular, sugary diminutive for Elizabeth, Elle, Ella, and the rest of the “El” clan, 0.3228% of baby girls were named Ellie in 2022, not beating out Elizabeth, who had -0.000288% more girls sharing the name, or Eleanor, with -0.00176% more. Ella was slightly higher than Ellie, by -0.001952%; and Eliana just below, with Ellie having -0.001359% more births. e
Lucy, the popular standalone, can be a nickname for Lucille, Lucilla, and Lucia(na), was the name worn by 0.2472% of girls; her longer forms did not rank in the top 100.
Sophie: just as independent as Mia or Lucy, Sophia’s diminutive featured with 0.1968%
of births, a far cry from the 0.6899% of girls named Sophia, a top five name.
Sadie not Sarah, is rising steadily, with 0.1837% of girls named Sadie in 2022, where Sarah came in with 0.1558% of births; the difference being -0.000144%.
Cora, a not-quite-considered nickname, made up 0.1709% of girls born this year, whereas her possible longer forms such as Coraline or Coral are unranked in the top 100.
Anna, another popular long form, is a potential nickname for Annalise, Annabella, and any other names with the prominent “anna” sounds. She ranked at 83 of 100, where 0.1650% of girls were named Anna.
Bella, meaning “beautiful” in Italian, is also a standalone for the quite popular Isabella, and fading Annabella. She makes up 0.1618% of girls born in 2022, where Isabella makes up 0.6535%; the difference between them is -0.000172%.
BOYS
Liam is the most popular boys’ name for this year, with 1.0977% of boys born this year being named Liam. This name outranks the percentage of girls bearing the #1 name, Olivia, (0.9288%) giving the name an American feel, though it began as an Irish name, a nickname for William (0.6054% were named William in 2022).
Jack has long been used as an average-Joe name, beginning as a nickname for John that is still used. 0.4770% of boys were named Jack in 2022, outranking John by -0.003815%.
Leo, a possible nickname for Leon or any Leo- names, yet a strong standalone, ranks at #22 with 0.4427% of newborn boys bearing the name Leo. His Italian counterpart Leonardo is at a difference of -0.00057%, yet 53 ranks.
Luke, a quick name to call out, is sometimes used just for that reason by boys named Lucas or Luca. Not eclipsing Lucas’ popularity, 0.3941% of boys were named Luke, whereas 0.5854% were named Lucas, 0.4187% called Luca, and the other variants such as Lucca or Lukas were unranked in the top 100.
Kai, sharp short form, stands alone but also as a shortened form of Malachi, Zakai, or any other “Kai” names. 0.2867% of boys were named Kai, while the other “Kai” names were unranked.
Eli, the zippy pet form of Elijah, Elio, Elias, et al., ranks at #69, where 0.2503% of boys born were named Eli. -0.000471% more boys were named Elijah than Eli, and -0.001732% called Elias as the longer form.
Theo joined the top 100 as the favorite pet form of Theodore, yet lower ranked than his parent name. 0.1939% of boys were named Theo, whereas 0.5771% were called Theodore.
Most Used:
Mia & Liam
Sources
Diminutive - Behind the Name
https://www.womansworld.com/posts/entertainment/reason-behind-common-nicknames-168200
Popular Baby Names
I hope this is useful, and feel free to let me know of any nicknames I have missed or gotten wrong! I will try and fix any wrong ones ASAP. 
Not asking you to answer, of course, just getting my wheels turning even more!
