Green’s Dictionary of Slang

1. to work as a prostitute, the image is a whore working from a given place, rather than walking the streets.

1969		‘Iceberg Slim’ Pimp 101: Up there, ain’t no walking. She can ‘flat back’.
1979		‘Iceberg Slim’ Airtight Willie and Me 38: You jive flat-backing zero bitch.
1990		S. Morgan Homeboy 7: Rings ’n’ Things [...] flatbacked for no man [i.e. pimp].
2003		Anchorage Press 9–15 Oct. 🌐 For fun, she flatbacks it down at the local sporting house, not so much for the extra coin as for the pungent smell of the waterfront trade.
2006		posting at www.perezhilton.com 25 Mar. 🌐 I bet your mother even hates you. Oh but wait maybe she flatbacks to help support your habit (fudge packing).

2. to have sex in the missionary position.

1972		Milner & Milner Black Players 91: Some [whores] will only ‘flat-back’ or ‘french’.
1991		W.T. Vollmann Whores for Gloria 48: He flatbacked Nicole.
2000		W.T. Vollmann Royal Family 496: And what have you heard it is said I do with cops [....] Flatback ’em?