E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A strong sinew running along the heel to the calf of the leg. The tale is that Thetis took her son Achilles by the heel, and dipped him in the river Styx to make him invulnerable. The water washed every part, except the heel covered with his mothers hand. It was on this vulnerable point the hero was slain; and the sinew of the heel is called, in consequence, tendo Achillis. A post-Homeric story
The Heel of Achilles. The vulnerable or weak point in a mans character or of a nation. (See above.)