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]