Pronounced "waf" as in "waffle." Its an acronym for "What A Fag" or "What A Faggot" and is said after some douche does or says something douche-like or gay. It can be shouted at some jerkoff who just said something politically correct or be muttered under your breath to your friend in class because your male teacher is wearing purple shorts.
Dude: Don't be gay, Manpuss.
Manpuss: Hey! Don't say that. That is offensive to me!
Bro: Why? Are you gay?
Manpuss: No, I am not.
Dude: W.A.F., man.
Bro: True. True.
Manpuss: Whatever, you racists.
Bro: W.A.F. for days.