inarticulate sound, attested from c. 1600; uh-huh, spoken affirmative (often ironic or non-committal) is recorded by 1894; negative uh-uh is attested from 1924.
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.
"be silent," 1560s, from a verb mum (Middle English mommen) "make silent" (c. 1400); "be silent" (mid-15c.), from mum, mom (late 14c.), "an inarticulate closed-mouth sound" indicative of unwillingness or inability to speak, probably imitative. As an adjective meaning "secret" or "silent" from 1520s. Phrase mum's the word is recorded by 1704.
c. 1200, mon, "lamentation, mourning, weeping; complaining, the expressing of complaints; a complaint; lover's complaint; accusation, charge," perhaps from an unrecorded Old English *mān "complaint," from mānan, a variant of mænan "complain, moan," also "tell, intend, signify" (see mean (v.1)); but OED discounts this connection. Meaning "long, low inarticulate murmur expressing grief or pain" is by 1670s, "with onomatopoeic suggestion" [OED].