BACKSIDE
buttocks, 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
(noun) 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?”
rear, backside, back end
(noun) the side of an object that is opposite its front; “his room was toward the rear of the hotel”
Source: WordNet® 3.1
Etymology
Noun
backside (plural backsides)
The back side of anything, the part opposite its front, particularly
The back side of an estate: the backyard and outbuildings behind a main house, especially (UK dialect, euphemism) an outhouse.
(euphemism) A person's buttocks.
(figuratively) The reverse or opposite of anything.
Synonyms
• (outhouse): backhouse (US); see also bathroom
• (buttocks): rear; see also buttocks
• (verso): See verso
Anagrams
• diebacks
Source: Wiktionary
Back"side`, n. Etym: [Back, a. + side. ]
Definition: The hinder part, posteriors, or rump of a person or animal.
Note: Backside (one word) was formerly used of the rear part or side
of any thing or place, but in such senses is now two words.
Source: Webster’s Unabridged Dictionary 1913 Edition