buttocks

n. (Anatomy) posterior pelvic area of the human body; nates, rear end

n. backside, one of two sections that make up the buttocks (Anatomy)

Bottom commonly refers to the human buttocks but also has other uses. The buttocks (anatomical nates, clunium, gluteus, regio glutealis) are rounded portions of the anatomy located on the posterior of the pelvic region of the apes, including humans and many other bipeds or quadrupeds.
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Buttocks /but·tocks/ the two fleshy prominences formed by the gluteal muscles on the lower part of the back. [more]Buttocks - Community and Resources

Noun
1. the fleshy part of the human body that you sit on; "he deserves a good kick in the butt"; "are you going to sit on your fanny and do nothing?"
(synonym) nates, arse, butt, backside, bum, buns, can, fundament, hindquarters, hind end, keister, posterior, prat, rear, rear end, rump, stern, seat, tail, tail end, tooshie, tush, bottom, behind, derriere, fanny, ass
(hypernym) body part
(part-holonym) torso, trunk, body

Noun
1. either of the two large fleshy masses of muscular tissue that form the human rump
(synonym) cheek
(hypernym) body part
(part-holonym) torso, trunk, body
(part-meronym) gluteus, gluteus muscle, gluteal muscle, glute

Synonyms and related words:
arse, ass, backside, beam, behind, bottom, breech, bum, butt, can, cheeks, croup, crupper, derriere, duff, fanny, fundament, haunches, heinie, hind end, hinder, hindquarters, keister, nates, posterior, prat, rear, rear end, rump, seat, secretiveness, stem, tail, tail end, tuchis, tush, tushy