Dictionary

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

Example: The beggar muttered words of thanks, as passersby dropped coins in his cup.

To speak softly and incoherently, or with imperfect articulations.

Example: The asylum inmate muttered some doggerel about chains and pains to himself, over and over.

To make a sound with a low, rumbling noise.

Example: April could hear the delivery van's engine muttering in the driveway.

Something that is muttered.

Example: I managed to catch a few of his mutterings.

A rumour.