kryptonitenoun [ U ]
/ˈkrɪp.tən.aɪt//ˈkrɪp.tə.naɪt/in stories and films, a fictional (= invented) substance that can hurt the character Superman, who is extremely strong and not hurt by most things
something that hurts or damages a person or thing that usually seems strong:
A slowing economy is kryptonite for the U.S. dollar.
His personal kryptonite comes in the form of a romantic entanglement.