taru-adjective, adverb taking the 'to' particle
Formal or literary term describing a situation, speech, or writing that lacks interest or excitement. Often used in the adverbial form 索然として.
彼の講演は索然として、聴衆の多くが居眠りを始めた。
His lecture was so dull that many in the audience started dozing off.
その小説の結末は索然たる
The ending of that novel was dry and uninspiring.
The kanji 索 means 'search' or 'lonely', and 然 is a suffix meaning 'state' or 'condition'. The compound 索然 likely derives from classical Chinese, where it described a desolate or uninteresting state. The exact historical derivation is uncertain, but the word has been used in Japanese literary contexts for centuries.