1(in the fictional word of the comic book, TV, and film character Superman) an alien mineral that has the property of depriving Superman of his powers.
- ‘the evil Lex Luthor diminishes Superman's powers with kryptonite’
- 1.1Something that can seriously weaken or harm a particular person or thing.‘flip-flopping on issues can be kryptonite to presidential candidates’
- ‘such uncertainties are kryptonite for the bond market’
1940s from Krypton, the name of Superman's home planet (kryptonite is usually depicted as a green mineral that came to earth from Krypton, following its destruction).