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.

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.