taru-adjective, adverb taking the 'to' particle
Describes a dignified, unhurried, and unruffled manner. Often used with と (悠然と) to modify verbs, or in the literary たる form (悠然たる).
彼は悠然と構えていた。
He remained calm and composed.
悠然たる態度で交渉に臨んだ。
He approached the negotiations with a composed demeanor.
Composed of 悠 (distant, leisurely) and 然 (like, in the manner of). The exact historical derivation is uncertain, but the compound follows a classical Chinese pattern for forming adjectives/adverbs.