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"buttocks," 1920, American English, from earlier British meaning "vulva" (1879), perhaps from the name of John Cleland's heroine in the scandalous novel "Fanny Hill or Memoirs of a Woman of Pleasure" (1748). The fem. proper name is a diminutive of Frances. The genital sense is still the primary one outside U.S., but is not current in American English, a difference which can have consequences when U.S. TV programs and movies air in Britain.
The buttocks; rump; ass: I can hardly sit down, my fanny is so sore
[1920+; fr earlier British fanny, ''vulva,'' perhaps fr John Cleland's 18th-century heroine Fanny Hill]