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"buttocks, rump," 1830s, of unknown origin.
The buttocks; rump; ass: A bunch of lazy guys sitting around on our duffs
[1830s+; origin uncertain; perhaps black slang for fud, ''buttocks,'' attested by 1785; perhaps fr duff, ''a (sailors') pudding boiled in a bag,'' which bag may have suggested a human fundament in shape; in this sense duff is a Northern pronunciation of dough]