To chill or stand at a spot (like a club or a street corner etc). To claim a spot. The term came from drug dealers on street corners, like light posts and street posts, and there you have it POST UP!

means you and someone are gonna fight, y'all about to get em up.

To incite a fight, or to call someone out.
used when people are mad and desire confrontation.

Basketball term where a big man like Shaq goes down low (near the basket, establishes a good position, pins his man with his body and asks for the ball.

To go hang or chill in no specific location.

A phrase commonly used by hipsters to indicate that they want to lean against (or 'post up') against a wall, preferably brick, to smoke a cigarette or drink a can of Pabst Blue Ribbon. Hipsters are led to believe that this is the cool thing to do. If there's no cigarette in hand, some hipsters replace the void with a can of PBR, the well known hipster beer. If they have nothing in hand, they fail at properly being a hipster. Generally, if caught in this scene, the hipsters would lift one leg up and rest the sole of their shoe against the side of the wall, completing their "cool" appearance. When suggested to 'post up,' a fellow hipster knows exactly what he/she is talking about and will usually agree to. It's the hipster thing to do.

While in the gym about to squat, bench, or deadlift you get your gear set next to the rack making sure that no one else takes it and that the person doing bicep curls realizes its time to get out.