Fallacious is a formal word that typically describes something, such as an argument or assumption, that is false or inaccurate and that tends to mislead or deceive others.

// The notion that disease is caused by malign spirits was known to be fallacious long before the advent of germ theory.

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"Bad legislation and fallacious arguments are threatening our most renowned educational institutions, which have made major contributions to our understanding of science and society and to our health and economic well-being, institutions that are the envy of the world." — Burton G. Malkiel, The Wall Street Journal, 19 Oct. 2022

It will come as no surprise that fallacious is related to the noun fallacy, meaning “delusion” or “falsehood.” Both words come from the Latin word fallacia, which in turn comes from fallere, meaning “to deceive.” (Other descendants of fallere in English include fail, false, and fault.) Fallacious arguments are a hot topic among philosophers, and some classic examples include the “ad hominem” fallacy, the “slippery slope” fallacy, and the “red herring” fallacy.