What is the meaning of muttering?

noun

• A low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech.
  • synonyms: mutter, murmur, murmuring, murmuration, mussitation
• A complaint uttered in a low and indistinct tone.
  • synonyms: grumble, grumbling, murmur, murmuring, mutter

verb

• Talk indistinctly; usually in a low voice.
  • synonyms: mumble, mutter, maunder, mussitate
• Make complaining remarks or noises under one's breath.
  • usage: "she grumbles when she feels overworked"
  • synonyms: murmur, mutter, grumble, croak, gnarl