kneecap

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To cripple by shooting in the legs, especially in the knees.

from The Century Dictionary.

• noun The bone capping the protuberance of the knee; the kneepan; the patella. See cut under knee-joint.
• noun Any covering for the knee, worn as a protection from injury either to the joint or to the clothing that covers it.
• noun Milit., same as genouillère.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A roundish, flattened, sesamoid bone in the tendon in front of the knee joint; the patella; the kneepan.
• noun A cap or protection for the knee.
• transitive verb to break the knees of, especially by shooting in the kneecap; -- often done by criminal or terrorist groups as a warning or punishment.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The flat, roundish bone in the knee.
• verb transitive To destroy the kneecaps of (a person), usually by shooting at the knees, as a punishment carried out by criminals or terrorists.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small flat triangular bone in front of the knee that protects the knee joint
• verb shoot in the kneecap, often done by terrorist groups as a warning