In Greek mythology, Achilles was the son of Peleus, a man, and Thetis, an immortal sea nymph. Thetis wanted her son to be immortal as well, so she dipped him in the river Styx. She held him by the ankle, and this was the only part of Achilles where he
could get hurt. Later, Achilles fought in the Trojan War and died from a wound from an arrow in his ankle.
How is the term "Achilles' heel" used today?