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]

The Free On-line Dictionary of Computing (19 January 2023):

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)


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