The phrase "Achilles' heel" originates from the story of Achilles in Greek mythology, specifically from his vulnerability due to his mother dipping him in the River Styx. Despite being invincible in battle, his heel remained unprotected, which ultimately led to his downfall. This concept symbolizes a critical weakness in a person's otherwise strong character. The tale imparts a moral lesson about the importance of recognizing and addressing one's vulnerabilities, as they can determine outcomes in life and conflict.