but·tock 
(bŭtək)
(bŭtək)n.
1.
a. Either of the two rounded prominences on the human torso that are posterior to the hips and formed by the gluteal muscles and underlying structures.
b. The analogous part of the body on certain mammals.
2. buttocks The rear pelvic area of the human body.
[Middle English, from Old English buttuc, strip of land, end; see bhau- in the Appendix of Indo-European roots.]