mum vs mammy

mum and mammy both are nouns.

Word	Noun	Adjective	Verb	Adverb
mum	Yes	Yes	No	No
mammy	Yes	No	No	No

As nouns, mum and mammy are synonyms defined as:

Other synonyms of mum include ma, mama, mamma, mom, momma, mommy, mumma, mummy.

mum (noun)	mammy (noun)
secrecy	informal terms for a mother
informal terms for a mother	an offensive term for a Black nursemaid in the southern U.S.
of China

mum (adjective)	mammy (adjective)
failing to speak or communicate etc when expected to