an/somebody's Achilles' heel

A weakness or vulnerability that can lead to permanent destruction or downfall. In Greek mythology, the hero Achilles was killed after being struck in the heel—the only weak spot on his body. Improper security measures were the failed company's Achilles' heel. I'm a good student, but I know I won't score high enough on the scholarship test because math is my Achilles' heel.

Fig. a weak point or fault in someone or something otherwise perfect or excellent. (From the legend of Greek hero Achilles, who had only one vulnerable part of his body, his heel; as an infant his mother had held him by one heel to dip him in the River Styx to make him invulnerable.) He was very brave, but fear of spiders was his Achilles' heel.

A fatal weakness, a vulnerable area, as in This division, which is rarely profitable, is the company's Achilles' heel. The term alludes to the Greek legend about the heroic warrior Achilles whose mother tried to make him immortal by holding the infant by his heel and dipping him into the River Styx. Eventually he was killed by an arrow shot into his undipped heel. [c. 1800]

A vulnerable or weak spot. The term is derived from the Greek myth of the hero Achilles, whose mother held him by the heel while dipping him into the River Styx to make him immortal. He eventually was killed by an arrow shot into his heel. The term became a literary metaphor about two centuries ago and remains current as a cliché.

A vulnerable spot that leads to a downfall. According to Greek mythology, anyone who was immersed in the River Styx, which marked the boundary of the underworld, became invulnerable. Thetis dipped her young son Achilles in the river, but she held him by his heel. Because her hand covered that part of his body, the water did not touch it and it became his one vulnerable spot. Achilles, who grew to become a great warrior, died during the Trojan War when an arrow struck his heel. Even though it's located in the same part of the body, don't confuse “Achilles' heel” with “Achilles tendon,” which connects muscles in your lower leg to your heel bone.