A weakness; a soft spot; a vulnerable spot

He's an excellent preacher but he would have more credibility and a larger following if beautiful women and money weren't his Achille's heel

È un ottimo predicatore, ma avrebbe più seguito e più credibilità se non avesse il suo tallone d'Achille nel denaro e nelle belle donne

Paul is a very fine man, but gambling is his Achille's heel and I'm afraid that sooner or later he will lose the shirt off his back

Paolo è un uomo in gamba, ma il gioco d'azzardo è il suo tallone d'Achille e ho paura che prima o poi perderà anche la camicia

In Greek mythology, Achilles, son of king Peleus and sea-goddess Thetis, was an invulnerable hero. All of his body was invulnerable, except for one heel; in fact, his mother dipped him in the river Styx – the river that encircles Hades – to make him invulnerable and immortal, but in doing so she held him by one heel, which was never touched by the water. During the Trojan war, with the help of goddess Minerva, he killed the Trojan hero Hector and continued violating his corpse, thus provoking the anger of the gods. Seeking revenge, Apollo told the secret of Achille's heel to Paris – Hector's brother and Achille's mortal enemy –, who killed him by shooting a poisoned arrow into his vulnerable heel