Meaning of “mutter” - Learner’s Dictionary

mutter

verb [ I, T ] uk us /ˈmʌtər/

B2 to speak quietly so that your voice is difficult to hear, often when complaining about something:

She walked past me, muttering to herself.
He muttered something about the restaurant being too expensive.
mutter noun [ C ]

(Definition of “mutter” from the Cambridge Learner’s Dictionary © Cambridge University Press)