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. [1913 Webster]
The convexity of a ship behind, under the stern. Mar. Dict. [1913 Webster]

top

OXFORD DICTIONARY

buttock, n. (usu. in pl.)
1 each of two fleshy protuberances on the lower rear part of the human body.
2 the corresponding part of an animal.

Etymology

butt ridge + -OCK

top

ROGET THESAURUS

buttock

Rear

N rear, back, posteriority, rear rank, rear guard, background, hinterland, occiput, nape, chine, heels, tail, rump, croup, buttock, posteriors, backside scut, breech, dorsum, loin, dorsal region, lumbar region, hind quarters, aitchbone, natch, natch bone, stern, poop, afterpart, heelpiece, crupper, wake, train, reverse, other side of the shield, back, rear, hind, hinder, hindmost, hindermost, postern, posterior, dorsal, after, caudal, lumbar, mizzen, tergal, behind, in the rear, in the background, behind one's back, at the heels of, at the tail of, at the back of, back to back, after, aft, abaft, astern, sternmost, aback, rearward, ogni medaglia ha il suo rovescio, the other side of the coin.

For further exploring for "buttock" in Webster Dictionary Online