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"be silent," 1560s, from Middle English mum, mom (late 14c.), inarticulate closed-mouth sound, indicative of unwillingness or inability to speak. As an adjective meaning "secret" from 1520s. Phrase mum's the word is first recorded 1704.
abbreviation of chrysanthemum, first attested 1915 in the jargon of gardeners.
Please keep quiet; don't say anything (1704+)