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1.
A sexually promiscuous woman who prefers the missionary position. Originally used in detective novels of the 1930s and 1940s as "flatbacker".
"My sister was the neighborhood flatback."
by tman March 13, 2004
 
2.
Having a skinny, bony, or non-portruding ass. Often referring to non-Black women's natural inability to grow curvacious behinds (particularly White women).
You ever date a white girl? Nah, I can't do a flat back.
by Bcolding June 25, 2008
 
3.
Someone who has a flatback as a result of laying on their back too much. Who are of a promiscious nature. Or simply, a female who is a sloppy, loose, trollup.
"man, that Deanna, she is such a flat back, I have seen her with 3 different guys this week!"

or

"Wow, why is he dating that flatback whore?"
by Elphaba. W. Witch July 12, 2008
 
4.
Flatback is an African American slang euphemism for, white girl. The etymology of the word is derived from another slang word, back, which in turn is a euphemism for female buttocks. Callipygous rondure is highly selected for with respect to the African American feminine beauty ideal. Thus, European women are seen as typically having flat butts or backs.
Damn, Jessica Biel isn't half bad, for a flatback.
by Hoopskidoodle August 21, 2009