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.