To speak quietly and unclearly, often so others can't understand.
From Middle English 'momelen,' which originally meant to chew with toothless gums, like how babies or very old people eat. The word literally described the mouth movements of someone without teeth trying to eat solid food. By the 1400s, it shifted to describe unclear speech that resembled those same lip and tongue movements.
When you mumble, you're literally making the same mouth movements that medieval people associated with toothless eating. The word captures that universal human experience of watching someone gum their food and thinking their speech sounds just as mushy and unclear.
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