taru-adjective, adverb taking the 'to' particle
noisy and lively; boisterous
Describes a scene or atmosphere that is bustling, clamorous, and full of energetic activity. Often used in literary or descriptive contexts with と (e.g., 喧然と).
祭りの会場は喧然としていた。
The festival venue was noisy and lively.
教室が喧然となる。
The classroom becomes boisterous.
The kanji 喧 means 'noisy' and 然 is a suffix forming adjectives/adverbs describing a state. The reading けんぜん is the on'yomi of the characters. The exact historical derivation is uncertain, but it follows a common pattern for literary Sino-Japanese adjectives.