Speaking unclearly or quietly so that people can't understand what you're saying.
From Middle English 'momble,' probably imitative in origin (the sound itself imitates unclear speech). Related to 'mum' (silent) and German 'mummen' (to grumble).
The word 'mumbles' is perfectly imitative—when you say it out loud, your mouth literally makes the same shapes it would if you were actually mumbling, which is why onomatopoeia words are so satisfying to say.
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