Source : Webster's Revised Unabridged Dictionary (1913)

Mumble \Mum"ble\, v. t.
   1. To utter with a low, inarticulate voice. --Bp. Hall.

   2. To chew or bite gently, as one without teeth.

            Gums unarmed, to mumble meat in vain. --Dryden.

   3. To suppress, or utter imperfectly.

Mumble \Mum"ble\, v. t. [imp. & p. p. {Mumbled}; p. pr. & vb. n.
   {Mumbling}.] [OE. momelen; cf. D. mompelen, mommelen, G.
   mummelen, Sw. mumla, Dan. mumle. Cf. {Mum}, a., {Mumm},
   {Mump}, v.]
   1. To speak with the lips partly closed, so as to render the
      sounds inarticulate and imperfect; to utter words in a
      grumbling indistinct manner, indicating discontent or
      displeasure; to mutter.

            Peace, you mumbling fool.             --Shak.

            A wrinkled hag, with age grown double, Picking dry
            sticks, and mumbling to herself.      --Otway.

   2. To chew something gently with closed lips.

Source : WordNet®

mumble
     v 1: talk indistinctly; usually in a low voice [syn: {mutter}, {maunder},
           {mussitate}]
     2: grind with the gums; chew without teeth and with great
        difficulty; "the old man had no teeth left and mumbled his
        food" [syn: {gum}]

Source : Free On-Line Dictionary of Computing

mumble
     
        1. Said when the correct response is too complicated to
        enunciate, or the speaker has not thought it out.  Often
        prefaces a longer answer, or indicates a general reluctance to
        get into a long discussion.  "Don't you think that we could
        improve LISP performance by using a hybrid reference-count
        transaction garbage collector, if the cache is big enough and
        there are some extra cache bits for the {microcode} to use?"
        "Well, mumble ... I'll have to think about it."
     
        2. Yet another {metasyntactic variable}, like {foo}.
     
        3. Sometimes used in "public" contexts on-line as a
        placefiller for things one is barred from giving details
        about.  For example, a poster with pre-released hardware in
        his machine might say "Yup, my machine now has an extra 16M of
        memory, thanks to the card I'm testing for Mumbleco."
     
        4. A conversational wild card used to designate something one
        doesn't want to bother spelling out, but which can be
        {glark}ed from context.  Compare {blurgle}.
     
        5. [XEROX PARC] A colloquialism used to suggest that further
        discussion would be fruitless.
     
        (1997-03-27)