"Kryptonite" is Superman's weakness. In the DC comics, kryptonite is the only weakness that Superman has. However, in this song, he is saying that this girl is his weakness. You will hear people use "kryptonite outside of discussing comic book heroes. Some will say "kryptonite" to refer to their own weaknesses. Ex: Water is her kryptonite. She doesn't know how to swim. Ex: Onions are his kryptonite because he is severely allergic.
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