The Collaborative International Dictionary of English v.0.48:

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.
      Often used in the plural -- see buttocks.

   Syn: cheek.
        [1913 Webster]

   2. (Naut.) The convexity of a ship behind, under the stern.
      --Mar. Dict.
      [1913 Webster]

WordNet (r) 3.0 (2006):

buttock
    n 1: either of the two large fleshy masses of muscular tissue
         that form the human rump [syn: buttock, cheek]