Sentence:  That's what you for—to help white folks keep niggers down. That's why he sent you to me. They be calling you <WSD>mammy</WSD> in a few years. You be running the whole house when the old man dies. 
Meaning of the word marked with <WSD> tags:  In the southern United States, a black nanny employed to look after white children; or in the antebellum South, a female slave who was close to the household and looked after the children.