mutter (v.)

early 14c., moteren "to mumble, utter words in a low tone with compressed lips," from a common PIE imitative *mut- "to grunt, mutter" (source also of Old Norse muðla "to murmur," Latin muttire "to mutter," Old High German mutilon "to murmur, mutter; to drizzle"), with frequentative suffix -er. Related: Muttered; muttering.

"a murmur or murmuring," 1630s, from mutter (v.).