taru-adjective, adverb taking the 'to' particle
uninteresting; dampening the mood; spoiling the fun
A yojijukugo describing something that is utterly uninteresting or that spoils the enjoyment of a situation. Often used in literary or formal contexts with たる or として.
彼の話は興味索然たるものだった。
His story was utterly uninteresting.
その冗談は場を興味索然とさせた。
That joke threw a wet blanket on the gathering.
A yojijukugo composed of 興味 (interest) and 索然 (desolate, dreary). The exact historical derivation is uncertain, but the combination conveys a complete lack of interest or a mood-killing quality.