mutterer

intransitive verb

1. to speak in low, indistinct tones without much movement of the lips, as in complaining or in speaking to oneself
2. to complain or grumble
3. to make a low, rumbling, threatening sound, as thunder

Origin: ME moteren, akin to Ger muttern, ult. < IE echoic base *mu- (see mope) > L muttire

transitive verb

noun

1. the act of muttering
2. something muttered; esp., a complaint or grumble