Achilles heel, a vulnerability or weakness. The expression comes from Greek mythology. Achilles was an almost invincible warrior, a fearsome fighter and all round superhero who fought in the Trojan war. He was the offspring of the nymph Thetis and the king of the Myrmidons. The Gods Zeus and Poseidon were keen to form a union with Thetis, but were warned that the offspring would be a greater man than his father! Zeus, being king of the gods, heeded the warning.
To make Achilles immortal, his mother Thetis dipped him in the river Styx, holding him by his heel, resulting in his eponymous weakness.
As we know, it helps to die young if you want to leave a good story behind you. Achilles chose wisely when offered a short glorious life over a long and undistinguished one. Read here about his part in the Trojan war.
He was shot through the heel by Paris (who started the Trojan war by stealing Helen from the Greeks, but to be fair he was manipulated by Aphrodite, Goddess of Love. But that’s another story.)
So the mighty Achilles is felled by an arrow to the heel….