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 proven to be fallacious — for now, anyway.

Fallacious comes ultimately from the Latin fallax, "deceptive." The word fallacious might describe an intentional deception or a false conclusion coming from bad science or incomplete understanding. A tween's assumption that anyone over 20 can't understand her situation would be fallacious; we have all been young once too.

Definitions of fallacious

adj containing or based on a fallacy

fallacious reasoning”
having no cogency or legal force

adj based on an incorrect or misleading notion or information

fallacious hope”
incorrect, wrong
not correct; not in conformity with fact or truth

adj intended to deceive

fallacious testimony”
deceitful, fraudulent
dishonest, dishonorable
deceptive or fraudulent; disposed to cheat or defraud or deceive