The Collaborative International Dictionary of English v.0.48:

Mutter \Mut"ter\, v. i. [imp. & p. p. Muttered; p. pr. & vb.
   n. Muttering.] [Prob. of imitative origin; cf. L. muttire,
   mutire.]
   1. To utter words indistinctly or with a low voice and lips
      partly closed; esp., to utter indistinct complaints or
      angry expressions; to grumble; to growl.
      [1913 Webster]

            Wizards that peep, and that mutter.   --Is. viii.
                                                  19.
      [1913 Webster]

            Meantime your filthy foreigner will stare,
            And mutter to himself.                --Dryden.
      [1913 Webster]

   2. To sound with a low, rumbling noise.
      [1913 Webster]

            Thick lightnings flash, the muttering thunder rolls.
                                                  --Pope.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mutter \Mut"ter\, v. t.
   To utter with imperfect articulations, or with a low voice;
   as, to mutter threats. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mutter \Mut"ter\, n.
   Repressed or obscure utterance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mutter
    n 1: a low continuous indistinct sound; often accompanied by
         movement of the lips without the production of articulate
         speech [syn: mutter, muttering, murmur, murmuring,
         murmuration, mussitation]
    2: a complaint uttered in a low and indistinct tone [syn:
       grumble, grumbling, murmur, murmuring, mutter,
       muttering]
    v 1: talk indistinctly; usually in a low voice [syn: mumble,
         mutter, maunder, mussitate]
    2: make complaining remarks or noises under one's breath; "she
       grumbles when she feels overworked" [syn: murmur, mutter,
       grumble, croak, gnarl]

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "mutter":
   air a grievance, aspirate, aspiration, bark, bated breath, bawl,
   beef, bellow, bellyache, bitch, blare, blat, blubber, boom, bray,
   breath, breathe, breathy voice, buzz, cackle, chant, chirp, clamor,
   complain, coo, crab, croak, crow, cry, dolorous tirade, drawl,
   drone, droning, exclaim, exhalation, flute, fret, fret and fume,
   fumble, fuss, gabble, gasp, gibber, gibbering, gripe, groan,
   grouch, grouse, growl, grumble, grunt, hiss, holler, howl, jabber,
   jeremiad, jibber, keen, kick, lament, lilt, little voice,
   lodge a complaint, low voice, maffle, maunder, maundering, moan,
   mouth, mouthing, muddle, mumble, mumbling, murmur, murmuration,
   murmuring, mussitate, muttering, outcry, pant, pipe, plaint,
   planctus, raise a howl, register a complaint, repine, roar, rumble,
   rumor, scold, scream, screech, shriek, sibilate, sigh, sing, snap,
   snarl, snort, sob, soft voice, sough, speak incoherently, splutter,
   sputter, squall, squawk, squeal, stage whisper, still small voice,
   susurrate, susurration, susurrus, swallow, take on, thunder,
   tirade, trumpet, twang, ululation, underbreath, undertone, wail,
   wail of woe, warble, whimper, whine, whisper, whispering, whistle,
   yap, yawp, yell, yelp, yowl



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

mutter
 vt.

    To quietly enter a command not meant for the ears, eyes, or fingers of
    ordinary mortals. Often used in ?mutter an incantation?. See also wizard
    .

    N


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

mutter

   To quietly enter a command not meant for the ears, eyes, or
   fingers of ordinary mortals.  Often used in "mutter an
   incantation".

   See also wizard.

   [Jargon File]

   (1995-02-06)