taru-adjective, adverb taking the 'to' particle
Describes a state of visible anger or indignation, often used in literary or formal contexts. Typically appears as 憤然として or 憤然と.
彼はその不当な扱いに憤然として抗議した。
He protested indignantly against the unfair treatment.
社長は部下の失態に憤然とした表情を見せた。
The company president showed an angry expression at his subordinate's blunder.
From 憤 (indignation) + 然 (a suffix forming adjectives/adverbs describing a state). The kanji 忿 is a variant of 憤.