SG Heritage Dictionary

Mumbling [Participle present tense] Webster's Dictionary 1828

Uttering with a low inarticulate voice; chewing softly or with a grumbling sound.

Mumbling [Adjective] Webster's Dictionary 1913

Low; indistinct; inarticulate.

Mumbling [Present participle and verbal noun] Webster's Dictionary 1913

Mumblingly [Adverb] Webster's Dictionary 1828

With a low inarticulate utterance. [Mumble and mutter are not always synonymous; mutter often expresses peevishness, which mumble does not.]