taru-adjective, adverb taking the 'to' particle
Describes a person's demeanor or attitude as remaining calm and unperturbed, often in situations where one might be expected to show emotion. Used with と as an adverb (平然と) or with たる/とした as an adjective.
彼は批判されても平然としていた。
He remained calm even when criticized.
平然たる態度で嘘をつく。
He tells a lie with a composed attitude.
Compound of 平 (flat, calm) and 然 (state, condition). The exact historical derivation is uncertain, but the combination conveys a state of being calm and composed.