Green’s Dictionary of Slang

1. an act of conventional heterosexual intercourse, usu. as offered by a prostitute, who lays ‘flat on her back’.

1992		T. Williams Crackhouse 120: Sexual activity ranges from ‘flat back,’ or missionary-position sex, to anal sex with many partners.
2000		W.T. Vollmann Royal Family 306: If you want a flatback or a blow, you just down your money in my hand. [Ibid.] 625: You can pay me twenty and I’ll give you a nice flatback.

2. attrib. use of sense 1, pertaining to intercourse, esp. when for money.

2001

J. Stahl Plainclothes Naked (2002) 110: It [i.e. a murder] went down at a flatback motel called the Pawnee Lodge.

3. a promiscuous woman; a prostitute.

2005

J. Stahl I, Fatty 165: The flatback [...] was known to have fornicated with anything in pants.