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 unquestioningly adhere to traditional policies when there is good reason for change can be regarded as a fallacious appeal to authority.

1. Containing or based on a fallacy: a fallacious assumption. 2. Tending to mislead; deceptive: fallacious testimony.

Definition of fallacious adjective in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more.

Feb 21, 2026 · Adjective fallacious (comparative more fallacious, superlative most fallacious) Characterized by fallacy; false or mistaken. Synonyms: erroneous, incorrect, wrong; see also …

Something fallacious is a mistake that comes from too little information or unsound sources. Predictions that the whole state of California will snap off from the rest of North America and float away have …

There are five meanings listed in OED's entry for the adjective fallacious, one of which is labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.

If an idea, argument, or reason is fallacious, it is wrong because it is based on a fallacy.

Fallacious definition: Containing or based on a fallacy.

Britannica Dictionary definition of FALLACIOUS [more fallacious; most fallacious] formal : containing a mistake : not true or accurate a fallacious [= false] set of assumptions fallacious [= misleading] …