시치미 떼다: to play dumb, to act like you don’t know, brazen out, to keep a stiff face

시치미: During Goryeo Dynasty, people used to hunt birds with pet falcons. They tied nametags on their falcons to prevent losing them. The nametag’s called 시치미. 떼다: take off, get rid of

→ Sometimes people would switch lost falcons’ nametags and take them as theirs and pretend to know nothing about it. In this sense, the meaning of ‘시치미를 떼다’ became ‘pretending to know nothing’, ‘playing dumb’, or ‘playing innocent’. (서울신문, ‘우리말여행’ 2009.03.10)

→ Pretending to know nothing about something, pretending to be innocent.