Originally (early 70s or so), a standard cliched response to the question "What's up?". Then, in the late 80s/early 90s, a new generation misunderstood the origin of the response so that it soon became the standard reply to similar questions, such as "How ya doin'?" or "What's goin' on?"
Dude 1: What's up, man?
Dude 2: You know, nothin' but the rent.
Dude 1: Hey, man, how you doin'?
Dude 2: Aw, nothin' but the rent, man.