mamma vs mammy

mamma

noun
  • The milk-secreting organ of female humans and other mammals which includes the mammary gland and the nipple or teat; a breast; an udder. 

  • An accessory cloud like a mammary in appearance, which can form on the underside of most cloud genera. 

mammy

noun
  • mamma; mother 

  • 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. 

How often have the words mamma and mammy occurred in a corpus of books? (source: Google Ngram Viewer )