Meaning

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 talk indistinctly; usually in a low voice

originFrom Middle English muteren, moteren, of imitative origin. Compare Low German mustern, musseln (“to whisper”), German muttern (“to mutter; whisper”), Old Norse muðla (“to murmur”). Compare also Latin muttīre, mutīre.

Etymology from Wiktionary, CC BY-SA 3.0