Short for the phrase "Fuck off wanker!"
Used when pissed and want somebody to either shut the fuck up, or get the fuck away from you
Jehovah's witness: *knocks on door*
Me: *opens door* THIS IS YOUR LAST WARNING! F.O.W. OR ILL KILL YOU!!!
Stephen: Wooo! red sox!
Me: F.O.W.! No one likes the fucking red sox!
Me: Julian, F.O.W. -.-