우물 안 개구리 (Umul an gaeguri)

Literally: A frog in a well

Example: “해외여행을 한 번도 안 해 본 그는 우물 안 개구리다.” — Never having traveled abroad, he can't see beyond the end of his nose.

English equivalent: Not to see beyond the end of one's nose

Origin: Traces to the "Autumn Floods" (秋水) chapter of the Chinese Daoist text Zhuangzi, in a story where the River God, having just marveled at his own river swollen to its widest by seasonal floods, is humbled upon finally reaching the open sea and realizing how small his own perspective had been by comparison. The sea spirit Ruo explains the River God's earlier limited view using the image of a frog that has spent its whole life at the bottom of a well: told about the ocean, the frog simply has no framework for understanding it, since its entire experience of the world has been bounded by the narrow walls and shallow water of the one well it has ever known. The Korean idiom keeps that exact image intact, describing anyone whose limited experience has left them unable to recognize how much larger and more complex the wider world actually is.