early 14c., momelen, "to eat in a slow, ineffective manner" (perhaps "to talk with one's mouth full"), probably frequentative of the interjection mum. The -b- is from 15c., unetymological. Meaning "to speak indistinctly in low tones" is from mid-14c. Transitive sense of "to utter in a low, inarticulate voice" is from mid-15c. Related: Mumbled; mumbler; mumbling.