Mammy is a slur used to describe Blacks in a derogatory way. Similar to Aunt Jemima. Stereotype of black women as nannies to white children, especially overweight black women - see "Gone With The Wind".
The word Mammy is a racial slur that has been used historically to degrade and dehumanize blacks. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to blacks. If you are unsure about whether or not it is appropriate to use the word Mammy, it is best to err on the side of caution and avoid using it in any situation.