means The kneecap is the small, movable bone (the patella) at the front of your knee joint that protects it and helps your leg straighten.

from A plain compound of "knee" + "cap" — "knee" descending from Old English "cneow," with cousins across the Germanic family and ultimately related to the same ancient root that gave Latin "genu" and Greek "gony." The "cap" half pictures the bone as a little cover sitting over the joint, the same homely logic that named the patella in Latin, literally "little dish." The grim verb sense — to "kneecap" someone by shooting that joint — is a 20th-century coinage from paramilitary punishment.