taru-adjective, adverb taking the 'to' particle
surprised; startled; with a look of surprise
Literary or classical word describing a sudden surprised expression or change in demeanor. Often used adverbially with と (e.g., 瞿然として). Rare in everyday speech.
He looked startled upon hearing the news.
He opened his eyes wide in surprise.
The exact derivation is uncertain; the kanji 瞿 is associated with looking around in surprise, and 然 is a suffix forming adjectives/adverbs. The word is a classical Chinese-derived expression.