Croupier

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early 18th century (denoting a person standing behind a gambler to give advice): French, from Old French cropier ‘pillion rider, rider on the croup’, related to Old French croupe (see croup2). Compare with crupper.

wiktionary

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Borrowed from French croupier.

etymonline

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croupier (n.)

"one who clears the winnings from the table in gambling," 1731, from French croupier "partner or assistant at the gaming table" (17c.), originally one who rides behind another, on the croup or "rump" of a horse (a word of Germanic origin); hence extended to any one who backs up another; a "second."