the back part of something, or a slang term for the buttocks.
Compound word from 'back' (Old English 'bæc') and 'side' (Old English 'sīde'). Originally literal meaning the back side of objects, it became a euphemistic term for buttocks.
Backside is the perfect example of a word that's both literal and crude—ancient people called the back part of things the 'backside' until they got embarrassed and started using it as a euphemism for butts!
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