(1):
(n.) Repressed or obscure utterance.
(2):
(v. i.) To utter words indistinctly or with a low voice and lips partly closed; esp., to utter indistinct complaints or angry expressions; to grumble; to growl.
(3):
(v. t.) To utter with imperfect articulations, or with a low voice; as, to mutter threats.
(4):
(v. i.) To sound with a low, rumbling noise.