From 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]