- a fictional radioactive substance that is the only thing capable of hurting or weakening the otherwise invulnerable superhero Superman: Exposure to Kryptonite nullifies Superman's powers and immobilizes him with pain.
- something that a person or thing cannot defend against or defeat; a nemesis or bête noire: The team has not found a way to stop the triple option attack, which has been their kryptonite over the last several years.Garlic oil is kryptonite for aphids.
