Synonyms for mutter

verb

1. To speak as if to oneself

   mumble, murmur, grunt, grumble, sputter, whisper, speak sotto voce (Italian), speak in an undertone, say to oneself, speak under one's breath, swallow one's words; see also mumble, utter.

2. To complain

   grumble, moan, groan, grouse*; see complain 1. See syn. study at mumble.