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.

To sound with a low, rumbling noise.

To utter with imperfect articulations, or with a low voice; as, to mutter threats.

Repressed or obscure utterance.

a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

a complaint uttered in a low and indistinct tone

talk indistinctly; usually in a low voice

make complaining remarks or noises under one's breath; "she grumbles when she feels overworked"