What does the first name jillian mean?

Jillian is Latin for "Down-Bearded Youth", down is the soft undercoat of feathers on a bird - especially the covering of a baby bird. When a young man first begins to grow hair on his body (beard, pubic hair, chest hair, underarm hair) it comes in the same texture as the head on the hair - soft and straight, like down. Thus a "down-bearded" man would be a very young one.