kryptonite
n.n. something that is very bad for you or makes you fail. It is a funny way to say that a person or thing is your worst enemy.
n. a substance or factor that is extremely harmful or debilitating to a specific entity. In modern slang, it refers to a person or thing that causes significant failure or distress.
His lack of sleep is his own personal kryptonite.
For many students, the final exam is their kryptonite because they struggle with the high-pressure environment.
While the team was otherwise dominant, their inability to handle the pressure of the championship final proved to be their ultimate kryptonite.
From a fictional radioactive element, kryptonite, that has a detrimental effect on the otherwise invulnerable character Superman. Apparently formed as krypton (“a chemical element, Kr”) + -ite.