taru-adjective, adverb taking the 'to' particle
cool; indifferent; nonchalant; calm
Formal literary term describing someone who remains composed and unruffled, often in a situation where others might react emotionally. Used with と as an adverb or in the たる form as an adjective.
He remained cool and indifferent even when criticized.
He approached the negotiation with a calm, nonchalant attitude.
From Chinese 恬然 (tiánrán), meaning 'calm' or 'tranquil'. The kanji 恬 means 'calm' or 'composed', and 然 is a suffix forming adjectives/adverbs. The exact historical derivation into Japanese is uncertain, but it is used as a literary term.