井底之蛙 -- Chinese
井の中の蛙 -- Japanese
우물 안 개구리 -- Korean
"Frog in a well" is a Chinese four-character idiom, or chengyu (成語), used to represent ignorance, or the limitations of a person's understanding or worldview. In addition to China, the idiom is also used in Japan, Korea, and other East Asian countries. In Japanese the idiom is often expanded into a longer kotowaza (Japanese proverb), "A frog in a well cannot comprehend the ocean." (井の中の蛙大海を知らず).
The proverb comes from a famous parable in the writings of the ancient Daoist philosopher Zhuangzi. A frog lives in a well, and thinking it is the most fantastic place in the world, tries to convince a passing turtle to come down and see his magnificent home. The turtle complies, but then humbles the frog by comparing the well unfavorably to the vastness of the ocean, which the frog has never seen.
Among other uses, in modern times the idiom "frog in a well" is often applied to academics and other people buried in their books, in a manner somewhat analogous to the English expression "ivory tower."