Short form for Fuck Ass Bitch. Normally an attempted insult from someone who is too stupid to actually make one up so they strung together 3 random swear words
Guy 1: Haha dude you are such a fag
Guy 2: Fuck you fucking .. Fuck ass bitch
Guy 1: Fuck ass bitch? What does that mean? Can you tell me because apparently I'm a fuck ass bitch? Instead of maybe fag I'm a FAB?