Not deriving from Scary Movie like everyone says, it derives from an oldBudweiser commercial. It is used to say "How are you?" "What are you doing?" oor just "What's up?"
Buddy:Wazzap?!?!
Guy: Nuhin' B. Watchin' the game, havin a Bud. Sup wit'chu?
Buddy: Nuhin', watchin' the game, havin' a Bud.
Guy: True, true.