An Achilles heel is a weakness in spite of overall strength, which 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. Synonyms.he use of “Achilles heel” as an expression meaning “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).