Green’s Dictionary of Slang

(US black pimp) a prostitute, esp. one who specializes in quantity rather than quality; also an honest prostitute (i.e. who delivers the promised sex and neither tricks nor robs her client).

1969		‘Iceberg Slim’ Pimp 162: You gonna starve to death Nigger, if she’s a chump flat-backer.
1972		Milner & Milner Black Players 42: A woman who makes her money by simply having intercourse with as many men as possible is known as a fiat-backer. She does not attempt to become involved with tricks in any way, will not go out to dinner or socialize with them, and will not play the role of a trick’s analyst or girlfriend. Women who make all their money flat-backing are said to be deficient in game.
1977		J.L. Dillard Lex. Black Eng. 89: A prostitute whose abilities do not include conning the John out of his scratch (i.e., money) is a flatbacker.
1979		‘Iceberg Slim’ Airtight Willie and Me 29: A cavalcade of tricks, flat backers, stuff players and thieves paraded past.
1989		H. Rawson Dict. of Invective (1991) 56: A flatbacker, a prostitute.
1990		S. Morgan Homeboy 7: He made a practice of sending to hell [...] flatbackers dipping his girls’ business.
2007		UGK ‘Gravy’ 🎵 I fuck the snow but I love a pro like flatbackers I’m a Cadillac’er.
2012		J. Robinson Gospel of the Game 19: These four whores were known [...] to be notorious thieves and flatbackers.