Definition from Wiktionary, the free dictionary
mutter (plural mutters)
- A repressed or obscure utterance; an instance of muttering.
- The prisoners were docile, and accepted their lot with barely a mutter.
- (in Indian restaurants) peas
instance of muttering
Derived terms 
- To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath.
- You could hear the students mutter as they were served sodden spaghetti, yet again, in the cafeteria.
- The beggar muttered words of thanks, as passersby dropped coins in his cup.
- 2012 June 28, Jamie Jackson, “Wimbledon 2012: Lukas Rosol shocked by miracle win over Rafael Nadal”, the Guardian:
- This set – the set of Rosol's life – was studded with aces and menacing ground-strokes that left Nadal an impotent spectator often muttering to himself and at the umpire regarding a perceived misdemeanour by his opponent.
- To speak softly and incoherently, or with imperfect articulations.
- The asylum inmate muttered some doggerel about chains and pains to himself, over and over.
- To make a sound with a low, rumbling noise.
- April could hear the delivery van's engine muttering in the driveway near the kitchens.
- (speak under one's breath): growl, grumble, mumble
- (speak incoherently): babble, mumble, murmur, ramble, stutter
- (make a low sound): growl, putter, rumble
- See also Wikisaurus:mutter
speak under one's breath
make a low sound
From German Mutter (“mother”).
mutter c (singular definite mutteren, not used in plural form)
- nut (for bolts)
Inflection of mutter
- a nut (for bolts)
Declension of mutter
mutter n (uncountable)
- mutter; obscure utterance