Achilles' Heel, the vulnerable part. It is said that when Thetis dipped her son in the river Styx to make him invulnerable, she held him by the heel, and the part covered by her hand was the only part not washed by the water. This is a post-Homeric story.

[Hanover] is the Achilles' heel to invulnerable England.--Carlyle.

(Sometimes Ireland is called the Achilles' heel of England.)

¶ Similarly, the only vulnerable part of Orlando was the sole of his foot, and hence when Bernardo del Carpio assailed him at Roncesvallês, and found that he could not wound him, he lifted him up in his arms and squeezed him to death, as Herculês did Antæos.