 A saying that comes from ancient times is Achilles' heel, which everyone's heard of. It means a weakness or vulnerability. You might say, he's a great runner, but that's his Achilles' heel. When he can't do long distances, for example. It has its origins in ancient Greek legend. Achilles was dipped into a river by his mother in order to give him a skin of armor and make him invulnerable to his enemies. But she held him by the heel, which didn't get covered by the water and became a weak point for him.