Considering the time period, the women ethnicity, and what history tells us about what happened to female slaves, it is what it seems to depict : the rape of a black woman.
It's hard to imagine someone painting this/that expression on her face and finding the subject matter acceptable. Or so I like to think? Maybe I'm naive. But if it's true, it's a great example of powerful art confronting viewers. Or I suppose that's true either way, for better or worse.