Sup

[Rihanna]

A term that cool people use because they are too damn lazy to say Whats up. (Too much effort involved there man.)

[Justin Bieber]

A contraction of the phrase whats up?

[Couch Potatoe]

A greeting, an abbreviation of Whats Up

[Sarah]

How everyone starts an IM convo after saying hey. It is then always followed by nm

[Tanya Shivari]

short version of [Wassup], [Whats up], or [what up]

[Becky]

The point to a sup is one trying to say hello, sup was derived from wassup, which is another variation of Whats up. Whats up isnt an inquiry but an informal way of saying hello, although it can at times be answered with an actual response (Ex 1.)
Many do in fact actually respond to the question with an answer but quickly disregard it and fray away from it... another way of responding to it would be (Ex 2.)

[Little_Girl]

Whats Up?

[Little_Girl]

[Sup] is something [cool people] say. It can be used as a greeting. People say [sup] when they dont feel like saying whats up because its to long. Also, people say sup because they get in the [habit] of saying sup so they just always say it. It can be used by girl or boy.