Part of speech: Noun
a complaint uttered in a low and indistinct tone
Part of speech: Noun
a low continuous indistinct sound
Example: often accompanied by movement of the lips without the production of articulate speech
Part of speech: Verb
talk indistinctly
Example: usually in a low voice
Part of speech: Verb
make complaining remarks or noises under one's breath
Example: she grumbles when she feels overworked