- The definition of fallacious is something based on an erroneous or mistaken belief.
When you make an argument based on a mistaken belief, this is an example of an argument that would be described as fallacious.
fallacious definition by Webster's New World
- containing a fallacy; erroneous: fallacious reasoning
- misleading or deceptive
- causing disappointment; delusive
Origin: Classical Latin fallaciosus
fallacious definition by American Heritage Dictionary
- Containing or based on a fallacy: a fallacious assumption.
- Tending to mislead; deceptive: fallacious testimony.
- fal·laˈcious·ly adverb
- fal·laˈcious·ness noun