an 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.

Someone's Achilles heel is the thing that causes problems for them, especially because it gives other people a chance to attack or criticize them. Horton's Achilles heel was that he could not delegate. The economy was from the start the Achilles heel of his regime. Note: This expression comes from the Greek myth in which the baby Achilles is dipped in the river Styx to protect him from being killed by an arrow. Because his mother held his heel to do this, his heel was not protected and he was killed by a poisonous arrow in it.