Kryptonite

google

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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).

wiktionary

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From a fictional radioactive element, kryptonite, that has a detrimental effect on the otherwise invulnerable character Superman. Apparently formed as krypton(“a chemical element, Kr”) +‎ -ite.

etymonline

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kryptonite (n.)

fictional substance in the "Superman" series, where it weakens the otherwise invulnerable hero, 1943; perhaps from elements of krypton (which is a gas) + meteorite.