Greek Mythology: Achillies' mother dipped him in the River Styx to make him immortal for protection. She did this by holding his heel so he wouldn't fall all the way in. So the only part of him that isn't immortal is his heel because it didn't enter the River Styx. So that's his only weakness. (Aka Achillies curse). <== Adding on to this, your Achilles tendon is named after this.