taru-adjective, adverb taking the 'to' particle
Describes a person's demeanor as calm and unshaken, especially under pressure. Often used in the adverbial form 泰然として or 泰然と.
He remains calm and composed no matter what.
He took the criticism calmly and let it slide.
泰然 describes staying calm and unshaken regardless of events, the opposite of 一喜一憂 where one's mood swings with every change.
From Chinese 泰然 (tàirán), meaning 'calm' or 'composed'. The characters 泰 (peaceful, calm) and 然 (state, manner) combine to express a state of unshakable calm.