Mutter
WordNet
noun
(1) A complaint uttered in a low and indistinct tone
(2) A low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
verb
(3) Make complaining remarks or noises under one's breath
"She grumbles when she feels overworked"
(4) Talk indistinctly; usually in a low voice
WiktionaryText
Etymology 1
From , from .