mum's the word
Webster's Dictionary (GCIDE)
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]