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
  1. adjective
    containing or based on a fallacy
    fallacious reasoning”
    synonyms: unsound
    having no cogency or legal force
  2. adjective
    based on an incorrect or misleading notion or information
    fallacious hope”
    incorrect, wrong
    not correct; not in conformity with fact or truth
  3. adjective
    intended to deceive
    fallacious testimony”
    synonyms: deceitful, fraudulent
    dishonest, dishonorable
    deceptive or fraudulent; disposed to cheat or defraud or deceive
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