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.
He remained calm even when criticized.
He tells a lie with a composed attitude.
平然 means staying calm and unruffled without necessarily implying wrongdoing, whereas のこのこ implies appearing brazenly in a situation where one should feel embarrassed.
平然 means 'calmly, unperturbed' and can lack the moral condemnation that ぬけぬけ carries when describing shameless acts.
平然 is a more literary word for looking unmoved or unruffled, while 平気 is the everyday word for feeling unbothered.
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.