# Achilles' heel

idiom · CEFR B2

Source: https://englishreference.com/idiom/achilles-heel
Updated: 2026-07-29

## Meaning

A single hidden weakness in someone or something otherwise strong.

## Examples

- Math has always been my Achilles' heel.
- The team was strong everywhere except in defence, which proved to be their Achilles' heel in the final.
- For all its engineering brilliance, the aircraft had an Achilles' heel: a single sensor whose failure could mislead the entire flight-control system.

## Origin

Achilles was born under a prophecy that he would die young, and his mother Thetis set out to cheat it. She carried the infant down to the underworld and dipped him into the black water of the River Styx, whose current made mortal flesh proof against any blade. She held him, as any mother would, by the heel. The water closed over every inch of him but the small patch her fingers covered, and there, unseen, the boy stayed as mortal as the day he was born. Achilles grew into the greatest warrior of his age. Through ten years of war at Troy he walked out of battles no other man could survive, until the whole Greek camp believed him untouchable. Then Paris, guided by the god Apollo, loosed a single arrow, and of all the places it might have struck, it found the one dry patch of skin on his heel. The invincible hero fell to the smallest of wounds. An Achilles' heel has meant the same thing ever since: the one hidden flaw that can bring down all the rest of a person's strength.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.