|Noah Webster's Dictionary|
1. (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.
2. (v. i.) To sound with a low, rumbling noise.
3. (v. t.) To utter with imperfect articulations, or with a low voice; as, to mutter threats.
4. (n.) Repressed or obscure utterance.
Int. Standard Bible Encyclopedia
mut'-er (haghah (Isaiah 8:19; Isaiah 59:3)): An onomatopoetic word, used of the growling of a lion (Isaiah 31:4), of the "mourning" of a dove (Isaiah 38:14), or of the human voice, whether speaking inarticulately (Isaiah 16:7) or articulately (Psalm 37:30, "The mouth of the righteous talketh of wisdom" compare Job 27:4 Proverbs 8:7, etc.). Hence, it is only the context that can give to haghah the meaning "mutter." No such meaning can be gathered from the context of Isaiah 59:3, and, in fact, the open shamelessness of the sinners seems to be in point. So the verse should be rendered, "Your lips have spoken lies, your tongue uttereth wickedness." In Isaiah 8:19 haghah describes the tone of voice used by the necromancers in uttering their formulas, "that chirp and that mutter." That this tone was subdued and indistinct is quite probable.
Burton Scott Easton
Mutter (6 Occurrences)
Job 27:4 surely my lips shall not speak unrighteousness, neither shall my tongue utter deceit. (See NAS)
Psalms 2:1 Why do the nations rage, And the peoples meditate a vain thing? (See JPS)
Psalms 115:8 Nor do they mutter through their throat, Like them are their makers, Every one who is trusting in them. (YLT)
Isaiah 8:19 When they tell you, "Consult with those who have familiar spirits and with the wizards, who chirp and who mutter:" shouldn't a people consult with their God? Should they consult the dead on behalf of the living? (WEB KJV JPS ASV DBY WBS YLT NAS RSV NIV)
Isaiah 59:3 For your hands are defiled with blood, and your fingers with iniquity; your lips have spoken lies, your tongue mutters wickedness. (Root in WEB KJV JPS ASV DBY YLT NAS RSV NIV)
Lamentations 3:62 The lips of those that rose up against me, and their muttering against me all the day. (Root in JPS NIV)