fan·ny \ˈfanē, -aan ̷ ̷\noun (-es) Etymology: from Fanny, nickname of Frances 1.slang Britain: billy III 2.alsofany\“\usually capitalized[fanny irregular (influence of the nickname Fanny) from First Aid Nursing Yeomanry; fany from First Aid Nursing Yeomanry]: a member of a British women's ambulance unit 3.: buttocks