Not deriving from Scary Movie like everyone says, it derives from an old Budweiser commercial. It is used to say "How are you?" "What are you doing?" oor just "What's up?"
1: Yo, what you doin?
2: Ya know, watching the game, drinking a Bud...
1: True, true
2: Whazaaaaaap!
1: Whazaaaaaap!
2: Whazaaaaaap!
<TRRRIIIINNNGGG>
2: Yo pick up the phone!
1: Hello...?
3: Whazaaaaaap!
1: Whazaaaaaap!
2: Whazaaaaaap!