My nickname is X, but it's short for Y, not Z

[CherryBomb50]

[name_m]Gilberto[/name_m]

[name_f][/name_f]

[name_f]My[/name_f] nickname is [name_f]Cyn[/name_f], but it’s not short for [name_f]Cynthia[/name_f], it’s short for ____.