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."