What is the meaning of Achilles' heel?

It is a person's point of weakness. Achilles, according to legend, was dunked in the River Styx by his mother Thetis when he was a baby. She wanted to make him immortal. While dunking him, she held him by the heel. This is the only body part that was not immortal. During the Trojan War, Paris shot him in the heel, killing him. His heel was his only weak point, hence his "Achilles Heel.'