Based on a mistaken belief or faulty reasoning; misleading or deceptive.


US English

UK English

Part of Speech



deceptive, misleading, false, untrue, incorrect, mistaken, flawed, invalid, spurious, specious


true, accurate, valid, sound, reliable, honest

Word Forms

Part of Speech Words
Noun fallaciousnesses, fallaciousness, fallacies, fallacy
Verb None
Adjective fallacious
Adverb None

Example Sentences

  • The argument he made was fallacious and did not hold up under scrutiny.

  • She fell for the fallacious claim that the product could cure all illnesses.

  • The advertisement made fallacious promises about the benefits of the product.

  • His belief in the conspiracy theory was based on fallacious information.


The word “fallacious” is used to describe something that is based on a mistaken belief or faulty reasoning, and is therefore misleading or deceptive. It can be used to describe arguments, claims, beliefs, or any other form of communication that is not supported by evidence or logical reasoning. The word has a negative connotation, suggesting that the fallacious statement or argument is not trustworthy or reliable.

The word “fallacious” comes from the Latin word “fallax,” which means “deceitful” or “treacherous.” The suffix “-ious” is commonly added to words to turn them into adjectives that describe a characteristic or quality of something.

The word “fallacious” can also be used in combination with other prefixes and suffixes to create new words with related meanings. For example, the prefix “non-” can be added to create the word “non-fallacious,” which means not misleading or deceptive, and is used to describe things that are true or accurate. The suffix “-ity” can be added to create the word “fallaciousness,” which refers to the quality of being fallacious.

Overall, the word “fallacious” is a useful word that describes a common phenomenon in human communication and reasoning. Its negative connotation reflects the harm and danger that can arise from false beliefs or misleading information, and it serves as a warning against irrationality and manipulation. As such, it is a valuable addition to any vocabulary.