To place a pistol or other firearm behind a persons kneecap and pull the trigger, obliterating the knee and crippling the victim. A common mob tactic.
The track star wouldn't pay his debts, so we knee capped him.
a thick bone under the skin in the middle of your leg, also used for kneeing people in the thigh (also known as a charlie or dead leg).
ooooow my knee cap
ill pop a knee in your leg