Green’s Dictionary of Slang

(US) the buttocks.

1944		D. Runyon Runyon à la Carte 116: I feel a distinct sensation of pain in my Francesca.
1952		(con. 1931) G. Fowler Schnozzola 150: She warbled the lyrics, ‘I’m taking no chances sitting on my Frances in the Great Indoors’.
1966		Trimble 5000 Adult Sex Words and Phrases.