An extremely annoying question that has (unfortunately) replaced "Hello" or "Hi" as the most popular form of casual greeting. For full effect, best delivered while walking past someone at about 90 m.p.h. so the person you're talking to has no time to respond, and in a tone that suggests you really don't give a shit about "what's up" with them.
Dude #1: "Hey, man. What's up?"
Dude #2: "Uh..." (what he managed to get out before Dude #1 was already too far away to hear the rest of what he was going to say)