Speaking in a way that is unclear or hard to understand, with words running together or spoken too quietly or quickly.
From Middle English 'momelen' or similar roots, possibly imitative of the mouth sounds made while speaking unclearly. Related to 'mum' (silent).
Mumbling might be imitative of the actual mouth movements—the 'mm' and 'mb' sounds require your lips to close, so the word phonetically demonstrates what mumbling does to speech, forcing the listener's mouth to mirror the unclear speaker.
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