Canard
Etymology
From French "canard," "duck." The sense of a hoax is usually traced to the old French expression "vendre un canard à moitié," "to half-sell a duck," an idiom for cheating or fooling someone. From that phrase "canard" came to mean an absurd or fabricated report, and English borrowed it whole to name a groundless rumor dressed up as news.
noun · /kuh-NARD/
A false or baseless story, rumor, or claim, especially one spread to deceive.
What it names
The durable falsehood that keeps circulating on reputation alone — the claim so often repeated that people stop asking whether it was ever true.
Example
The claim that the two founders had never met is a canard that resurfaces every few years.
Related idea
A canard is a specific false claim in circulation; sophistry is bad reasoning, which can spread a canard but is a defect of argument rather than of fact.