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

“The beggar muttered words of thanks, as passersby dropped coins in his cup.”

To speak softly and incoherently, or with imperfect articulations.

“The asylum inmate muttered some doggerel about chains and pains to himself, over and over.”

To make a sound with a low, rumbling noise.

“April could hear the delivery van's engine muttering in the driveway.”

Something that is muttered.

“I managed to catch a few of his mutterings.”

A rumour.