The Collaborative International Dictionary of English v.0.48:

mammy \mam"my\ (m[a^]m"m[y^]), n.; pl. mammies
   (m[a^]m"m[i^]z).
   A child's name for mamma, mother.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mammy
    n 1: an offensive term for a Black nursemaid in the southern
         U.S.
    2: informal terms for a mother [syn: ma, mama, mamma,
       mom, momma, mommy, mammy, mum, mummy]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "mammy":
   amah, ayah, dam, dry nurse, foster mother, genetrix, ma, mam, mama,
   mater, materfamilias, maternal ancestor, matriarch, mom, mommy,
   mother, mummy, nanny, nurse, nursemaid, nurserymaid, old lady,
   old woman, sitter, stepmother, the old woman, wet nurse