mum (interj.)

"be silent," 1560s, from a verb mum (Middle English mommen) "make silent" (c. 1400); "be silent" (mid-15c.), from mum, mom (late 14c.), "an inarticulate closed-mouth sound" indicative of unwillingness or inability to speak, probably imitative. As an adjective meaning "secret" or "silent" from 1520s. Phrase mum's the word is recorded by 1704.

mum (n.1)

abbreviation of chrysanthemum, by 1915 in the jargon of gardeners.

mum (n.2)

pet word for "mother," 1823, short for mummy (see mamma). In British sociology, used from 1957 in reference to "the working class mother as an influence in the lives of her children." Also sometimes a vulgar corruption of madam or ma'am.

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Definitions of mum from WordNet
mum (n.)
of China;
Synonyms: florist's chrysanthemum / florists' chrysanthemum / Dendranthema grandifloruom / Chrysanthemum morifolium
mum (n.)
informal terms for a mother;
Synonyms: ma / mama / mamma / mom / momma / mommy / mammy / mummy
mum (n.)
mum's the word
mum (adj.)
failing to speak or communicate etc when expected to;
Synonyms: silent