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.
Mutter is a noun and a verb.
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"
He responded to our attempt to get him out of bed with several minutes of incoherent muttering.