Derriere

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late 18th century: French, literally ‘behind’.

etymonline

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derriere (n.)

"backside, arse," colloquial, 1774, from French derrière "back part, rear," originally an adverb, "behind, behind the back" (12c.), from Late Latin deretro, from Latin de "from" (see de-) + retro "back" (see retro-). In italics until 20c.