To speak with the lips partly closed, so as to render the sounds inarticulate and imperfect; to utter words in a grumbling indistinct manner, indicating discontent or displeasure; to mutter
talk indistinctly; usually in a low voice grind with the gums; chew without teeth and with great difficulty; "the old man had no teeth left and mumbled his food
If you mumble, you speak very quietly and not at all clearly with the result that the words are difficult to understand. Her grandmother mumbled in her sleep He mumbled a few words `Today of all days,' she mumbled. Mumble is also a noun. He could hear the low mumble of Navarro's voice. to say something too quietly or not clearly enough, so that other people cannot understand you mutter (From the sound)
grind with the gums; chew without teeth and with great difficulty; "the old man had no teeth left and mumbled his food"