In Greek mythology, Achilles’ heel was said to be his only vulnerable spot.
Achilles was the greatest Greek warrior in the Trojan War tradition and the central hero of much of Homer’s Iliad. Later writers developed the story that his mother, the sea nymph Thetis, tried to make him invulnerable by dipping him in the River Styx. She held him by one heel, leaving that spot unprotected.
The Iliad itself does not tell this River Styx story, and Achilles does not die within Homer’s poem. Later traditions say that Paris killed him during the Trojan War, often with an arrow guided by Apollo. These later accounts helped establish the heel as the famous weak point.
Today, “Achilles’ heel” means a person’s critical weakness. The expression is based on later mythological tradition rather than a complete account given by Homer.