"mutters" in the noun sense
1. mutter, muttering, murmur, murmuring, murmuration, mussitation
a low continuous indistinct sound often accompanied by movement of the lips without the production of articulate speech
2. grumble, grumbling, murmur, murmuring, mutter, muttering
a complaint uttered in a low and indistinct tone
"mutters" in the verb sense
1. mumble, mutter, maunder, mussitate
talk indistinctly usually in a low voice
2. murmur, mutter, grumble, croak, gnarl
make complaining remarks or noises under one's breath
"she grumbles when she feels overworked"
Source: WordNet® (An amazing lexical database of English)
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