Emily Dickinson Lexicon

MUT'TER, v.i. [L. mutio, muttio, and musso, mussito; allied perhaps to muse, – which see.]

1. To utter words with a low voice and compressed lips, with sullenness or in complaint; to grumble; to murmur. Meantime your filthy foreigner will stare, / And mutter to himself. Dryden.
2. To sound with a low rumbling noise. Thick lightnings flash, the muttering thunder rolls. Pope.