Contraction of the greeting "What's up?" which is a contraction of "What is up?" If you're really pressed for time, greet someone with a "[sup]"?. In any case, the greeter is rhetorically asking you what is going on in your life. Roughly synonymous with hello but less hip.
> Hipster1: Wussup?
> Hipster2: [Nuttin]'.
> Hipster1: [Word].
> Hipster2: [Word is bond].