知らん顔 (feigned ignorance)

Set phrase 知らん顔, literally 'unknowing face,' meaning to pretend not to know, to feign ignorance, or to pointedly ignore. The 知らん inside is a fossilized negative contraction of 知らない. Surfaces both as a bare noun (知らん顔をする 'put on a not-knowing face,' 知らん顔で 'with a not-knowing face') and as the verb phrase 知らん顔をする. Both cover the same idiomatic feign-ignorance / cold-shoulder meaning.