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]