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mum's the word
From The Collaborative International Dictionary of English v.0.48:
Mum \Mum\, a. [Of imitative origin. Cf. Mumble.] Silent; not speaking; as, to keep mum. --Thackeray. [1913 Webster] The citizens are mum, and speak not a word. --Shak. [1913 Webster] mum's the word keep this a secret; don't tell anybody. [PJC]