Origin: English, Irish
Meaning: a medieval English feminine form of Julian, derived from Julius, which either means “downy-bearded” or else is related to Jupiter, composed from elements dyeus meaning “shine” or “sky” and pater “father”.
It’s also an Anglicized form of Gaelic Mac Gileáin meaning “son of Gileán”, the latter derived from personal name Gealán, a diminutive of geal meaning “bright, white”.
Variants: Jillian, Jill
Though a lot of people would pronounce this the same is Jillian, I’ve always pronounced it with a hard g- like Gilbert.
Interesting fact: Apparently this used to be a unisex name once upon a time, although I don’t know how popular it was for boys. I found a male character with the name, however, named Gilliam B. Loeb, a character in the Batman universe.