Source : Webster's Revised Unabridged Dictionary (1913)

Kneecap \Knee"cap`\, n.
   1. (Anat.) The kneepan.

   2. A cap or protection for the knee.

Source : WordNet®

kneecap
     n : a small flat triangular bone in front of the knee that
         protects the knee joint [syn: {patella}, {kneepan}]
     v : shoot in the kneecap, often done by terrorist groups as a
         warning
     [also: {kneecapping}, {kneecapped}]