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Synonyms for Backside (same or very similar meaning)

WordNet sense 1 (the side of an object that is opposite its front):
rear, back end

WordNet sense 2 (the fleshy part of the human body that you sit on):
behind, bottom, bum, buns, buttocks, can, derriere, fanny, hindquarters, posterior, rump, seat, stern, tail, tush, arse, fundament, keister, nates, prat, tooshie, tush, tail end, hind end

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