noun
a mineral or substance that weakens or has a detrimental effect on someone or something, especially a fictional material that is the ultimate weakness of a particular superhero
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Scientists have identified kryptonite as the mineral that weakens Superman's powers.
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Dude, did you know that kryptonite is like Superman's weakness?
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That math test was my kryptonite, I totally bombed it.
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For many athletes, injuries are their kryptonite, hindering their performance on the field.