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Something that presents a particular threat to one that is otherwise powerful: Immigration reform turned out to be the senator's kryptonite.
[After Kryptonite, a fictional radioactive mineral (the presence of which weakens the superhero Superman) appearing in Superman comics from 1949 onwards : Krypton, the home planet of Superman where Kryptonite was once found before being scattered in the universe by the planet's destruction (named by American comics writer Jerry Siegel (1914-1996), perhaps after the element krypton) + -ite.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
informal something that renders a person or thing helpless: football movies are box-office kryptonite.
[C20: from the fictional substance to which the hero is vulnerable in the comic strip Superman]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014