Idiom : Achilles' heel
Meaning : To refer someone's weekness or flaw.
Story : Achilles was a famous warrior. When he was born, his mother dipped him to the river Styx . She did this to make him invincible(nothing could harm him).Unfortunately his mother held him by the heel. As a result, the water didn't touch his foot and his feet became vincible part of his body.Ultimately, he died by an arrow pierced his heel.
Example: Kavita is very intellegent but her Achilles' heel is pressure. She does not perform under pressure.