Where does “Narcissus” come from?

Narcissus comes from Ancient Greek Νάρκισσος, a name of unknown ultimate origin.

Narcissus (English): A youth who spurned the love of Echo and fell in love with his own reflection in a pool: in some versions of the myth, he drowned trying to reach it, while in others he sat fixated until a god took pity and transformed him into a flower