kryptonite (n.)

inert gaseous element, 1898, coined by its discoverers (Sir William Ramsay and Morris W. Travers) from Greek krypton, neuter of adjective kryptos "hidden" (see crypt); so called because it remained undiscovered for so long and was so difficult to find. Scientific American (July 9, 1898) announced it as "the discovery of yet another element."