An Achilles' heel is a weakness that can lead to someone's downfall. Achilles' heel comes from a Greek myth where his mother tried to make him invincible. A weakness in Achilles' heel made him vulnerable, showing how a small flaw can affect great strength. Learn the meaning and origin of the phrase " Achilles heel ", which refers to a person's weak point or vulnerability. See how it is used in literature, songs, and sentences with examples and analysis. Achilles Heel idiom means a person's point of greatest vulnerability or weakness. Check out synonyms, example sentences, questions, etc. Achilles heel definition: 1. a small problem or weakness in a person or system that can result in failure: 2. a small…. Learn more.