When something is your "Achilles' heel," it's a fault or weakness you have, despite overall strength, that can potentially cause failure. The weakness may be physical: "He is a star quarterback, but his injury-prone throwing arm is his Achilles' heel."Or it might be emotional or mental: "She was a good writer, but her Achilles' heel was that she was a terrible speller."
This expression comes from Greek mythology, specifically a guy named Achilles. Achilles' mother dipped him in the river Styx as an infant, an act that bestowed upon him extreme strength and immortality. He became a great champion — the best Greek fighter during the Trojan War — until the fateful day when Trojan prince Paris took aim and speared him in the heel with an arrow, causing him to bleed to death. But wasn't he immortal? Make that almost. When his mother dipped him in the river as a baby, she held him by one heel, which thus wasn't bathed in the river's magical waters and became the only part of his body that was unprotected [sources: Mythagora]. Oops!