expression, godan verb (-u)
have a bitter experience; get into trouble; get hurt; feel pain
Idiomatic expression meaning to suffer an unpleasant or painful experience, often as a consequence of one's actions. The 'bitter experience' nuance is central.
See also: 痛い目 · 目にあう (めにあう)
そんなことをすると痛い目にあうよ。
If you do that, you'll get into trouble.
彼は若い頃、痛い目にあってから変わった。
He changed after going through a bitter experience when he was young.