In Greek mythology, Achilles is made invulnerable by being dipped in the river Styx by his mother, Thetis. However, his mother held him by the heel, and eventually Achilles was felled by an arrow that hit this unprotected part. Today the term Achilles' heel refers to any inherent weakness.
The term Achilles' tendon refers to the portion of the body below the calf and is sometimes referred to as the Achilles' heel.
