Fallacious (adjective)

1. Containing a fallacy or error in reasoning.
2. Based on a mistaken or false belief.

Origin:

From latin fallax (deceitful, misleading), from fallere (to deceive).

Examples:

1. The argument was fallacious, as it relied on false assumptions.
2. The conclusion was fallacious, as it did not follow from the premises.
3. The reasoning was fallacious and didn't hold up to scrutiny.
4. His argument was fallacious and it was easy to see the flaws in his logic.
5. The evidence presented was fallacious and could not be used to support the claim.