speaking indistinctly and complainingly
I could not understand what it was that you were muttering about.
Muttering to oneself is a complex vocal tic
After the fight with her boyfriend, Olivia was left muttering arguments and comments under her breath.
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Muttering is not typically considered onomatopoeia. Onomatopoeia refers to words that phonetically imitate or resemble the sound they describe, such as "buzz" or "hiss." While muttering suggests a low, indistinct sound, it doesn't mimic a specific noise in the same way that onomatopoeic words do. Instead, it describes a manner of speaking rather than a sound itself.
Uttering, muttering, fluttering, cluttering, and sputtering.
To mumble words indistinctly or in a low tone, often as if talking to oneself.
The muttering was illegible.Nobody could decipher the illegible scribbles.
The crew has been muttering about mutiny, sir.
"Muttering retreats"
Mutter is a noun and a verb.
He responded to our attempt to get him out of bed with several minutes of incoherent muttering.