an Achilles heel

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.
See also: heel

an 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.
See also: Achilles, heel

an Achilles heel

a person's only vulnerable spot; a serious or fatal weakness.
In Greek mythology, the nymph Thetis dipped her infant son Achilles in the water of the River Styx to make him immortal, but the heel by which she held him was not touched by the water; he was ultimately killed in battle by an arrow wound in this one vulnerable spot.
1998 Times The inclination to outlaw that of which it disapproves…is, if not the cloven hoof beneath the hem of Tony Blair's Government, certainly its Achilles heel.
See also: Achilles, heel
References in periodicals archive ?
While the opening match may traditionally be an Achilles heel, it is a shoulder that is concerning Wales at the moment - that of Lions centre Jonathan Davies.
Well, I suppose that everyone has an Achilles heel (if you will excuse the pun).
They should have had two penalties against Valencia and, though the Spaniards played some lovely passing football, with Silva particularly dangerous, they have an Achilles heel.
There will always be some cast of characters looking for an Achilles heel.
He continued: "Every great player always had an Achilles heel.
An Achilles heel for many firms is inventory control, as Cooley Godward discovered.