(in the fictional word of the cartoon, TV, and film character Superman) an alien mineral that has the property of depriving Superman of his powers. Kryptonite is a fictional material that appears primarily in Superman stories. Kryptonite is a fictional material that appears primarily in Superman stories. My definition: A man/woman that calls someone their Kryptonite means that they are their weakness. 4 days ago · Meaning of kryptonite in English. Kryptonite definition is - a substance that causes the comic-book character Superman to become weak when he is exposed to it—often used to describe.