What is the meaning of Mutter?

A repressed or obscure utterance; an instance of muttering.

To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath.

To speak softly and incoherently, or with imperfect articulations.

To make a sound with a low, rumbling noise.

Peas.

mommy, mummy, ma

old woman

missus

nut (that screws onto a bolt)

a nut (for bolts)

a nut (for bolts)

a nut (for bolts)

mutter (muttering, utterances under one's breath, often angry)

Source: wiktionary.org