adverb taking the 'to' particle, taru-adjective
chatteringly; prattling on
Describes the manner of talking incessantly or idly, often with a soft or murmuring quality. Used as an adverb with と or as a taru-adjective in literary contexts.
彼女は喃喃と愚痴をこぼしていた。
She was chattering on with her complaints.
老人たちが喃喃と昔話に花を咲かせている。
The old folks are prattling on, reminiscing about the old days.
Implies grumbling or muttering to oneself, whereas 喃喃 suggests idle talk directed at others.
Reduplication of 喃 (nan), an onomatopoeic element representing murmuring or soft speech. The exact historical derivation is uncertain, but the form is used in literary Japanese to evoke gentle, continuous chatter.