The Collaborative International Dictionary of English v.0.48:

kneecap \knee"cap`\, n.
   1. (Anat.) A roundish, flattened, sesamoid bone in the tendon
      in front of the knee joint; the patella; the kneepan.
      [1913 Webster]

   2. A cap or protection for the knee.

The Collaborative International Dictionary of English v.0.48:

kneecap \knee"cap\ v. t.
   to break the knees of, especially by shooting in the kneecap;
   -- often done by criminal or terrorist groups as a warning or
   punishment.

   Note:

   Syn: knee-cap.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

kneecap
    n 1: a small flat triangular bone in front of the knee that
         protects the knee joint [syn: patella, kneecap,
         kneepan]
    v 1: shoot in the kneecap, often done by terrorist groups as a
         warning; "They kneecapped the industrialist"