You've heard this phrase a thousand times:
모르는 척 (해)
(mo-reu-neun cheok (hae))
It means: Pretend you don't know
The grammer **verb**+는 척 = to pretend to **verb** ^^~
Listen at 1:33:
She says "없는 척, 모르는 척" which means "pretend it's not there, pretend you don't know"
IU uses it to in Twenty Three:
(listen at 0:55 to 0:57)
She says "덜 자란 척해도" which means "even if I pretend to be immature"