to shoot someone with a handgund turned to the side

Originally, a kneecap. The phrase "bust a cap" originated in Northern Ireland several years before entering urban gangsta culture, and referred to the nonlethal (but fantastically painful and disabling) practice of firing a firearm at close range into the crook of the knee. The practice served, and in some areas continues to serve, primarily as an intimidation tactic, and the victim becomes a hobbling reminder of what happens to "the enemy."

refers to the ignition cap at the base of a cartridge for a firearm. "bust a cap" means break the cap by pulling the trigger, setting off the explosion that propels the bullet

Term used for the cap of a mushroom, usually one of the psychedelic nature.

insulting someone in a playful manner

a blowjob, the act of getting a blowjob "getting cap"

a word used to describe finishing something, originating from college students saving the bottlecaps of handles of liquor. To cap something is to finish it off, preferably with some sort of class. You can do this with bottles, women, drugs, and all sorts of other shit.

For example, finishing a handle involves drinking the last bit, but capping a handle involves saving the cap and throwing the bottle at someone. (classy as fuck)

Can be used in place of almost any word.