Fanny
late 19th century: of unknown origin.
wiktionary
19th century. Ultimately from Fanny, pet form of the name Frances. [1] Compare dick, John Thomas, mickey.
The British naval slang sense derives from Fanny Adams. Tins of mutton introduced as rations were not liked by the sailors and were taken by them to contain the butchered remains of Fanny Adams who had been brutally murdered and dismembered. The tins were re-used for eating from and cooking with. [1]
etymonline
fanny (n.)
"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.
