Definition: If a person has an Achilles’ heel, it means that they have a fatal weakness or vulnerable spot.
Origins: If you have seen Troy, the 2004 movie with Brad Pitt as Achilles, you already know the origins of this popular saying. For those who do not race to the theater for a Brad Pitt movie, the birth of this saying comes out of Greek mythology. The legend has it that the heroic warrior Achilles’ mother, Thetis, wanted her son to be immortal. Before Achilles’ birth, Thetis received a prophecy about her son’s future death in battle. Because of this, she wanted to counteract this foretelling by trying to make him immortal. When Achilles was born, Thetis held onto her son’s heel and dipped him in to the river Styx. The Styx is the mythical river that separate earth from Hades or the Underworld. As Achilles grew, he became a great warrior. Homer discusses Achilles and his role in the Trojan War in his epic poem, the Iliad. Homer did not mention Achilles’ death later in the war, but the legend states that Achilles met his death by an arrow into his heel. In fact it was the same heel his mother held onto when she attempted to make Achilles immortal.
The Achilles tendon was named after this legend.
Ammer, Christine. The American Heritage Dictionary of Idioms. Boston: Houghton Mifflin Company, 1997, 3.