Is it ensure, insure, or assure?
early 14c., moteren "to mumble," from a common PIE imitative *mut- "to grunt, mutter" (cf. 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.
1630s, from mutter (v.).