Etymology
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backside (n.)

"the rear part of anything," c. 1400, from back (adj.) + side (n.). In the specific sense of "rump of an animal, buttocks" it is recorded by c. 1500.

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Definitions of backside

backside (n.)
the side of an object that is opposite its front;
Synonyms: rear / back end
backside (n.)
the fleshy part of the human body that you sit on;
Synonyms: 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 wordnet.princeton.edu