Green’s Dictionary of Slang

of lesbians, to rub their bodies together for sexual stimulation.

1888–94		‘Walter’ My Secret Life (1966) I 200: The two girls on top of each other I thought a baudy amusment and did not believe until after years that flat fucking was practicable, and practised, with sexual pleasure. [Ibid.] VIII 1510: Chapter V Two years back. ― Harriet’s lust. ― At a B***s**s lapunar. ― Cunt inspections. ― The way ladies go up stairs. ― A large clitoris. ― Flat fuckers.
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		B. Rodgers Queens’ Vernacular.
2006		at www.amateurmansion.com 🌐 Licking and flatfucking their neighbouring girlfriends, and even involving in some real gang bang with their husband’s best friends!