Baidu's explanation is:
Mammy: Mom.
Appellation, ancient Chinese.
(1) old woman [grandma; Grandma]
The grandmother in this shop dressed up in flowers, packed a box of gifts and held them in both hands, all the way to Miaoguan Store. -"Two Moments of Surprise"
(2) Mother: the common name of mother.
Then I opened the door and his grandmother died. -"Flower and Moon Mark"
(3) The general term for elderly women: it also refers to [wet nurse].
It is also the mother inside, and the mother and father outside follow. -Children's Heroes
In the novel A Dream of Red Mansions, the wet nurse is called an old maid.
(4) In some places: the wet nurse is grandma.
(5) Wu dialect: address your father's sister, such as a northerner's aunt.
(6) Modern Chinese: addressing female practitioners (nuns) in western churches.
References:
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