“An Achilles’ heel” means a vulnerable spot.
In the Iliad, Homer’s marvelous legend, Achilles, the hero of the story, was the son of a king, Peleus, and of the sea-goddess, Thetis, and a great-grandson of no less than.
Zeus himself, The account that Homer gives of the boyhood of so remarkable a being is reasonably prosaic; that is, he was at least brought up on land, even though his tutors did include a centaur. But later writers than Homer felt compelled to provide greater details.
Thus, according to one account, the real reason why Achilles was fearless in battle was because he knew that everyone was powerless to hurt him. That was because his mother had dipped him in the river Styx, the river that encircles Hades, and had thus made him invulnerable.
He would have been living yet, perhaps, but the god Apollo, who was no friend, knew that Thetis had slipped, she had held Achilles by the heel when she dipped him and had neglected to get that heel wet.
Apollo whispered that secret to Paris, mortal enemy of Achilles, who deliberately aimed an arrow at this unprotected heel, the one spot that was vulnerable, and thus caused the death of the hero.
Incidentally, the tendon leading upward from the rear of the heel is even today called “Achilles’ tendon.”