Have you ever heard of the Greek hero Bophades?
He was one of the heroes who fought in the Trojan War. His story is very similar to the story of Achilles.
When he was a child, his mother held him by the groin and dipped him in the river Styx, as to make him invincible in battle. However, just like Achilles, he had a weak spot.
Because his mother held him by the groin, this was where he became vulnerable.
Just as in the case of Achilles, this was his heel.
So you may have heard of Achilles' heel, but I bet you've never heard of Bophades nuts.