Definitions of mutter:

• noun:   a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
• noun:   a complaint uttered in a low and indistinct tone
• verb:   make complaining remarks or noises under one's breath
• verb:   talk indistinctly; usually in a low voice
• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #10862)