unconcerned air; nonchalant manner; acting as if nothing happened
A rare, literary set phrase describing a person's demeanor when they pretend nothing is wrong or act casually to hide their true feelings. Often used in narrative or classical contexts.
He left the room with an unconcerned air.
She kept the secret by putting on a nonchalant manner.
何食わぬ顔 is a common idiom for acting innocent or as if nothing happened, similar in meaning but more colloquial than さあらぬ体.
Kana-kanji mixed spelling; the kanji 体 is used for 'appearance' or 'manner'.
Full kanji spelling, very rare and literary.
Derived from classical Japanese 然有らぬ (sa aranu, 'not being so') + 体 (tei, 'appearance'). The exact historical development is uncertain, but it is a fossilized literary expression.