The Collaborative International Dictionary of English v.0.48:

Mumble \Mum"ble\ (m[u^]m"b'l), v. t.
   1. To utter with a low, inarticulate voice. --Bp. Hall.
      [1913 Webster]

   2. To chew or bite gently, as one without teeth.
      [1913 Webster]

            Gums unarmed, to mumble meat in vain. --Dryden.
      [1913 Webster]

   3. To suppress, or utter imperfectly.
      [1913 Webster] Mumbledy peg

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Peace, you mumbling fool.             --Shak.
      [1913 Webster]

            A wrinkled hag, with age grown double,
            Picking dry sticks, and mumbling to herself.
                                                  --Otway.
      [1913 Webster]

   2. To chew something gently with closed lips.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mumble
    n 1: a soft indistinct utterance
    v 1: talk indistinctly; usually in a low voice [syn: mumble,
         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: mumble, gum]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "mumble":
   aspirate, aspiration, bark, bated breath, bawl, bellow, bite,
   blare, blat, blubber, boom, bray, breath, breathe, breathy voice,
   buzz, cackle, champ, chant, chaw, chew, chew the cud, chew up,
   chirp, chomp, coo, crow, drawl, drone, droning, exclaim,
   exhalation, flute, fumble, gabble, gasp, gibber, gibbering, gnash,
   gnaw, grind, growl, grunt, gum, hammer, hiss, jabber, jibber, keen,
   lilt, limp, little voice, low voice, maffle, masticate, maunder,
   maundering, mouth, mouthing, muddle, mumbling, munch, murmur,
   murmuration, murmuring, mussitate, mutter, muttering, nibble, pant,
   pipe, roar, rumble, ruminate, rumor, scream, screech, shriek,
   shuffle, sibilate, sigh, sing, snap, snarl, snort, sob, soft voice,
   speak, speak incoherently, splutter, sputter, squall, squawk,
   squeal, stage whisper, stammer, still small voice, stumble,
   stutter, susurrate, susurration, susurrus, swallow, talk, thunder,
   trumpet, twang, underbreath, undertone, utter, verbalize, vocalize,
   voice, wail, warble, whine, whisper, whispering, yap, yawp, yell,
   yelp



The Jargon File (version 4.4.7, 29 Dec 2003):

mumble
 interj.

    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. [MIT] Expression of not-quite-articulated agreement, often used as an
    informal vote of consensus in a meeting: ?So, shall we dike out the COBOL
    emulation?? ?Mumble!?

    3. Sometimes used as an expression of disagreement (distinguished from
    sense 2 by tone of voice and other cues). ?I think we should buy a VAX.
    ? ?Mumble!? Common variant: mumble frotz (see frotz; interestingly, one
    does not say ?mumble frobnitz? even though ?frotz? is short for
    ?frobnitz?).

    4. Yet another metasyntactic variable, like foo.

    5. When used as a question (?Mumble??) means ?I didn't understand you?.

    6. 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.?

    7. A conversational wild card used to designate something one doesn't want
    to bother spelling out, but which can be glarked from context. Compare 
    blurgle.

    8. [XEROX PARC] A colloquialism used to suggest that further discussion
    would be fruitless.


The Free On-line Dictionary of Computing (30 December 2018):

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
   glarked from context.  Compare blurgle.

   5. [XEROX PARC] A colloquialism used to suggest that further
   discussion would be fruitless.

   (1997-03-27)