* Quietly: The words are barely audible.
* Unclearly: The words are mumbled, and it's difficult to make out what is being said.
* Possibly secretly or resentfully: Muttering often suggests a lack of confidence or a reluctance to speak openly.
Here are some examples:
* "She muttered something about being tired and went to bed."
* "The angry customer muttered under his breath about the terrible service."
* "He muttered a curse as he tripped over the rug."
"Muttered" can also be used as a noun, referring to the act of speaking in a low voice. For example:
* "The meeting ended in a flurry of muttered complaints."