taru-adjective, adverb taking the 'to' particle
Obsolete literary word used almost exclusively in the adverbial phrase 恬として, meaning 'calmly', 'with indifference', or 'nonchalantly'.
See also: 恬として
彼は批判されても恬として気にしなかった。
Even when criticized, he remained nonchalant and didn't mind.
恬として騒がず、静かに座っていた。
He sat quietly, calm and unruffled.
平然 is a more common word for 'calm' or 'unperturbed', while 恬 is an obsolete literary term.
泰然 means 'composed' or 'self-possessed', often with a nuance of dignity, whereas 恬 emphasizes indifference or nonchalance.
The kanji 恬 is of Chinese origin, combining 心 (heart/mind) and 甜 (sweet) as a phonetic component, suggesting a sweet or calm state of mind. The exact historical derivation in Japanese is uncertain; the word is now obsolete outside the fixed expression 恬として.