painful experience; hard lesson
Often used in the phrase 痛い目にあう (to have a painful experience, to learn something the hard way).
そんなことをすると痛い目にあうよ。
You'll be sorry if you do that.
彼は痛い目を見て、やっと自分の間違いに気づいた。
He finally realized his mistake after a painful experience.
Formed from 痛い “painful” and 目 in its idiomatic sense “experience or fate,” as also seen in expressions describing an unpleasant experience.