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]

The Collaborative International Dictionary of English v.0.48:

Mum \Mum\, interj.
   Be silent! Hush!
   [1913 Webster]

         Mum, then, and no more.                  --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mum \Mum\, n.
   Silence. [R.] --Hudibras.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mum \Mum\, n. [G. mummere, fr. Christian Mumme, who first brewed
   it in 1492.]
   A sort of strong beer, originally made in Brunswick, Germany.
   --Addison.
   [1913 Webster]

         The clamorous crowd is hushed with mugs of mum. --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mum
    adj 1: failing to speak or communicate etc when expected to;
           "the witness remained silent" [syn: mum, silent]
    n 1: of China [syn: florist's chrysanthemum, florists'
         chrysanthemum, mum, Dendranthema grandifloruom,
         Chrysanthemum morifolium]
    2: informal terms for a mother [syn: ma, mama, mamma,
       mom, momma, mommy, mammy, mum, mummy]
    3: secrecy; "mum's the word"