Wordology, Achilles Heel

In Greek mythology, Achilles was the son of an immortal goddess named Thetis and a mortal man named Peleus. When Achilles was born, Thetis thought she could make him immortal by submerging him into the River Styx. The legend says Thetis held Achilles by his heel when she dipped him into the river. Therefore, his heel was not touched by the magical waters of the River and it remained mortal and thus vulnerable. Achilles went on to become a hero in the Trojan War, but was killed toward the end of the war when he was shot in the heel by an arrow.
Since then, the term "Achilles' heel" has come to mean a vulnerability or weakness in an otherwise strong person. For example, a fighter might be brought down by his Achilles' heel, such as pride, or overconfidence.

The Achilles tendon is the largest and strongest tendon in the body, stretching from the calf muscles down to the heel. It is strong, but prone to injury. Doctors speculate this is because of its limited blood supply and the fact that it frequently has high tension placed upon it, especially during walking and running.