means To speak in a low, indistinct voice, often complaining or grumbling under your breath so you're heard but not quite understood.

from From Middle English 'moteren,' an echoic word — meaning it imitates the very sound it names, that low 'mut-mut-mut' rumble. It's related to a family of murmuring words across Germanic and Latin tongues (compare Latin 'muttire,' to mutter or mumble), all of them likely born from the same instinct: the human noise for speech that barely escapes the mouth.