From WordNet (r) 2.0 (August 2003) [wn]:
backside
n 1: the side of an object that is opposite its front; "his room
was toward the rear of the hotel" [syn: rear, {back
end}] [ant: front]
2: 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?" [syn: buttocks, nates,
arse, butt, 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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Backside \Back"side`\ (b[a^]k"s[imac]d`), n. [Back, a. + side.]
The hinder part, posteriors, or rump of a person or animal.
[1913 Webster]
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.
[1913 Webster]