fanny (n.)

fem. proper name, from French, from Old French Franceise (Modern French Françoise), fem. of Franceis (see Francis).

British slang word with varied senses, not all of them certainly connected; see Partridge, who lists two noun uses: "female pudenda" (c. 1845), which might be back-slang from fan, shortening of fanny (in the British sense); and "nothing," in prostitutes' slang from c. 1940; a verbal use, a euphemism for fuck (v.) in oaths, imprecations, expletives (as in naff off), 1959, "making it slightly less obvious than eff" [Partridge]; and an adjective naff "vulgar, common, despicable," which is said to have been used in 1960s British gay slang for "unlovely" and thence adopted into the jargons of the theater and the armed forces.