Plural of backside; the rear or bottom part of something, or a person's buttocks.
From Middle English 'bak' (back) + 'side' (side). The compound emerged in the 15th century to describe the rear portion of any object or the posterior of a person's body.
The word 'backside' became a preferred euphemism for talking about butts in polite society—it's a perfect example of how English speakers create indirect words to discuss body parts we find embarrassing.
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