masc. proper name, from French François, from Old French Franceis "noble, free," as a noun "a Frenchman, inhabitant of Ile-de-France; the French language," from Late Latin Franciscus, literally "Frankish;" cognate with French and frank (adj.).
"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.