An Achilles’ heel is a deadly weakness in spite of overall strength, that can actually or potentially lead to downfall. While the mythological origin refers to a physical vulnerability, idiomatic references to other attributes or qualities that can lead to downfall are common.
In Greek mythology, when Achilles was a baby, it was foretold that he would die young. To prevent his death, his mother Thetis took Achilles to the River Styx, which was supposed to offer powers of invulnerability, and dipped his body into the water. But as Thetis held Achilles by the heel, his heel was not washed over by the water of the magical river. Achilles grew up to be a man of war who survived many great battles. But one day, a poisonous arrow shot at him was lodged in his heel, killing him shortly after. Still, Achilles is remembered as one of the greatest fighters who ever lived.
The death of Achilles was not mentioned in Homer's Iliad, but appeared in later Greek and Roman poetry and drama concerning events after the Iliad, later in the Trojan War. In the myths surrounding the war, Achilles was said to have died from a heel wound which was the result of an arrow—possibly poisoned—shot by Paris.
Classical myths attribute Achilles' invulnerability to a treatment of Ambrosia and burning away of his mortality in the house fire except on the heel, with which he was held by his mother Thetis. Peleus, his father, discovered the treatment and angered Thetis, who fled into the sea. Achilles was placed in the care of Chiron. (Demeter attempted a similar treatment on Demophon, or possibly Triptolemus.)
According to a myth arising later, his mother had dipped the infant Achilles in the river Styx, holding onto him by his heel, and he became invulnerable where the waters touched him—that is, everywhere except the areas of his heel that were covered by her thumb and forefinger. It is not clear how the waters of the Styx, which silenced the gods for nine years, could confer immortality; or how Thetis could gain access to them; or how Peleus would accidentally discover the project.
The use of “Achilles’ heel” as an expression used for “area of weakness, vulnerable spot” dates only to 1840, with implied use in Samuel Taylor Coleridge's "Ireland, that vulnerable heel of the British Achilles!" from 1810 (Oxford English Dictionary).
The large and prominent tendon of the gastrocnemius and soleus muscles of the calf is called the tendo achilleus or Achilles tendon. It is often believed in popular culture that the hero was therefore killed by being shot through this structure. However, as tendons are notably avascular, such an injury is unlikely to be fatal. However, in the myth the arrow had been covered in the blood of the Hydra, which was supposedly toxic.
The anatomical basis of Achilles' death is more likely to have been injury to his posterior tibial artery behind the medial malleolus, in between the tendons of the flexor digitorum longus and the posterior tibial vein. This area would have been included in Thetis' grip.
See also 
- Achilles tendon
- Balder in Norse mythology
- Duryodhana in the Mahabharatha
- Esfandiyar in the Shahnameh
- Siegfried in the Nibelungenlied
- Ovid, Metamorphoses 12.580–619
- See P. J. Heslin, The Transvestite Achilles: Gender and Genre in Statius’ Achilleid, Cambridge: Cambridge University Press: 2005, 166–169.
- Apollonius, Argonautica 4.869–872
- Statius, Achilleid 1.122f.,269f,480f.
- OED online