noun, noun or participle which takes 'suru', intransitive verb
feigned ignorance; pretending not to know; ignoring
Acting as if one does not know or recognize someone or something, often deliberately. Used in phrases like 知らん顔をする.
彼は私を見ても知らん顔をした。
He pretended not to recognize me even though he saw me.
困っている人がいても、みんな知らん顔で通り過ぎた。
Even though someone was in trouble, everyone passed by pretending not to notice.
noun, noun or participle which takes 'suru', intransitive verb
unconcerned air; indifference; nonchalance
An attitude of not caring or being indifferent, often shown by a blank or unconcerned expression. Can overlap with sense 1 but emphasizes lack of interest rather than deliberate pretense.
彼女は何を聞かれても知らん顔で座っていた。
She sat there with an unconcerned look no matter what she was asked.
大事な話なのに、彼は知らん顔を決め込んでいる。
Even though it's an important matter, he is acting indifferent.