Source : Webster's Revised Unabridged Dictionary (1913)

Buttock \But"tock\, n. [From {Butt} an end.]
   1. The part at the back of the hip, which, in man, forms one
      of the rounded protuberances on which he sits; the rump.

   2. (Naut.) The convexity of a ship behind, under the stern.
      --Mar. Dict.

Source : WordNet®

buttock
     n : either of the two large fleshy masses of muscular tissue
         that form the human rump [syn: {cheek}]