Dictionary definition · grumble
noun
- a loud low dull continuous noise"they heard the rumbling of thunder"
- a complaint uttered in a low and indistinct tone
verb
- show one's unhappiness or critical attitude"He scolded about anything that he thought was wrong"
- make complaining remarks or noises under one's breath"she grumbles when she feels overworked"
- to utter or emit low dull rumbling sounds"he grumbled a rude response"
- make a low noise"rumbling thunder"
Source: Open English WordNet (CC BY 4.0).