One of my top Welsh girl names is Hafren - the Welsh form of Sabrina. But it annoys me that Hafren/Sabrina have an unknown meaning. So, I did some research! 
Here’s what I found:
Hafren is a modern Welsh form of the Old Welsh Habren [Source: Hafren] and Sabrina is a Latinisation of Habren [Source: Sabrina].
Habren was orginially Sabren in Proto-Celtic [Source: Index of Celtic Elements, pg.132] and composed of the elements Sa- and Bren.
Sa- could be from sáil meaning “heel”, sail meaning “willow” [pg.200], or sam/samh meaning “summer” [pg.201].
Bren is possibly from pren meaning “tree” [pg.189], bre/bryn meaning “hill” [pg.46], or a corruption of bran meaning “raven” [See: Brangaine].
Summary of possible meanings of Sabrina:
“heel tree”, “heel hill” or “heel raven”
“willow tree”, “willow hill” or “willow raven”
“summer tree”, “summer hill” or “summer raven”
I think “willow tree” sounds the most plausible, closely followed by “summer raven”. The rest don’t make much sense and seem a bit chaotic, especially the “heel…” ones 
What does everyone else think? Am I crazy and seeing a meaning that isn’t there or do you agree that this could possibly be the origin of the lost meaning? Or does anyone have any sources for other possible meanings I’ve missed?
(Note: The Arabic name Sabrina/Sabreena originates from a different root to the Proto-Celtic name)