fallacious

Fallacious is the derived adjective from the noun fallacy (a mistaken or deceptive idea), so fallacious means deceptive or misleading.

Arguments, reasoning, and assumptions are most often characterized as fallacious: She spent all of her time refuting the fallacious arguments of the previous speaker.

Writers commonly begin sentence with “It’s fallacious” to introduce someone else’s mistaken idea: It’s fallacious to say that improved security would have prevented this accident.