Noah Webster's Dictionary 1. (v.) 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. 2. (v.) To chew something gently with closed lips. 3. (v. t.) To utter with a low, inarticulate voice. 4. (v. t.) To chew or bite gently, as one without teeth. 5. (v. t.) To suppress, or utter imperfectly.

Multi-Version Concordance Mumble (1 Occurrence) Isaiah 29:4 You will be brought down, and will speak out of the ground. Your speech will mumble out of the dust. Your voice will be as of one who has a familiar spirit, out of the ground, and your speech will whisper out of the dust. (WEB NIV)