From WikipediaMeaningA person's weak spot
OriginIn the myths surrounding the Trojan War, Achilles died from a heel wound as the result of an arrow fired by Paris.
According to a myth arising later, his mother, Thetis, 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 but the areas covered by her thumb and forefinger -- implying that only a heel wound could have been his downfall.
The use of "Achilles' heel" (or "Achilles heel") as an English expression for "area of weakness, vulnerable spot" dates only to 1855 (Merriam-Webster), or, in the form "heel of Achilles," 1810 (OED: Coleridge, "Ireland, that vulnerable heel of the British Achilles".)
The Achilles' heel as a singular weakness is a cliche in superhero comics.
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