People who have read the comic books or watched the movies featuring the exploits of Superman know that he was born on Krypton. When the planet exploded, kryptonite or the radioactive material from it, was hurled into space. Superman’s enemies discovered that kryptonite was the only thing that could be used to either hurt or kill him.
In everyday contexts, the word ‘kryptonite’ is used to refer to someone’s weakness or something that can be used to hurt someone who is strong. It has more or less the same meaning as ‘Achilles heel’.
Many tennis buffs believe that Nadal is Federer’s kryptonite.
When I was in school, Hindi was my kryptonite.
She is my kryptonite.