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1.
What's up? What's going on?

Typically used as an actual question, rather than as a greeting (sup yo!)

Depending on tone of voice, may also be used in a state of anger, surprise or upset.
Frat 1: wohaa what's loose?!

Frat 2: You're mama's what's loose!

Frat 1: No I mean, what is going on here?!

Frat 2: Yeah, and I mean your mama's what's going on here!
by vuck April 03, 2011