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. "Her assumption that anyone that old — over 20 — could understand her tween dilemma was fallacious; her sister had been young once too."
