pre-noun adjectival
casual; indifferent; nonchalant
Pre-noun adjectival (rentaishi) describing a person's demeanor or expression as unconcerned, unruffled, or as if nothing is wrong. Often used in phrases like さあらぬ体, さあらぬ顔, or さあらぬようす.
彼はさあらぬ顔で嘘をついた。
He told a lie with a nonchalant look on his face.
さあらぬ体を装って、彼女は部屋を出ていった。
Pretending to be indifferent, she left the room.
平然 is more about being calm and composed, often in the face of something that would normally cause agitation. さあらぬ emphasizes a deliberate pretense of indifference or casualness.
何食わぬ顔 means 'as if nothing happened' and is very similar in usage. さあらぬ is slightly more literary and less common in modern speech.
Derived from the classical Japanese phrase 然にあらぬ (sa ni aranu), literally 'not being so', which evolved into a fixed pre-noun adjectival meaning 'as if it were not so' or 'pretending nothing is the matter'.