taru-adjective, adverb taking the 'to' particle
Literary or formal word describing a proud, triumphant, or elated manner. Often used in the adverbial form 昂然と.
彼は勝利を確信して昂然と胸を張った。
Convinced of victory, he puffed out his chest in triumph.
昂然たる態度で演壇に立った。
He stood at the podium with a proud demeanor.
Composed of 昂 (high-spirited, elated) and 然 (a suffix forming adjectives/adverbs describing a state). The exact historical derivation is uncertain, but the combination conveys a state of being proudly elevated.