Language & Mythology: Achilles' Heel

Achilles was a hero in Greek mythology. He was an important person in the Trojan War, and in Homer's Iliad. Achilles was the son of Peleus and the nereid Thetis.
The meaning of the phrase Achilles' heel is "a weak or vulnerable factor".

The legend of Achilles has it that he was dipped into the river Styx by his mother Thetis in order to make him invulnerable. His heel wasn't covered by the water and he was later killed by an arrow wound to his heel.

In the Trojan War, Achilles was the best Greek warrior-hero. He killed Hector, but was killed by Paris, who shot his heel with a poisoned arrow.

Although the legend is ancient, the phrase Achilles' heel wasn't picked up in English until the 19th century. It is used as a metaphor for vulnerability, as in the earliest citation, an essay by Samuel Taylor Coleridge in The Friend; a literary, moral and political weekly paper, 1810:
"Ireland, that vulnerable heel of the British Achilles!"