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   A sort of strong beer, originally made in Brunswick, Germany.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"

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "mum":
   anaudic, aphasic, aphonic, breathless, brief, brusque, close,
   close-tongued, closemouthed, concise, curt, dumb, dumbfounded,
   dumbstricken, dumbstruck, economical of words, inarticulate,
   indisposed to talk, laconic, ma, mam, mama, mammy, mater, mom,
   mommy, mummy, mute, old woman, quiet, short, silent, snug,
   sparing of words, speechless, stricken dumb, taciturn, terse,
   tight-lipped, tongue-tied, tongueless, unloquacious, untalkative,
   voiceless, word-bound, wordless