when a male fully clothed rubs his cock (tip of his penis, or cap) against any part of another person male or female as if by accident or brushing his crotch against them so that they can feel the penis. this is a tradition in a town called riverhead, and mostly preformed by a man named joe who works in said town.
lori, i was sitting in my seat on a plane and this fuck putting his bag in the overhead just capped me !
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."
I told him I'd bust a cap in his ass, but it turns out he didn't have an asscap after all, so I let him go.