A canard is a false or unfounded story that spreads widely, often repeated so many times that some people start believing it despite having no real evidence. The word comes from French, where it literally means “duck,” but in English it specifically means a deliberate hoax or baseless rumor.

When someone calls a claim a canard, they're saying it's actively misleading and deliberately false. For example, the old story that people in medieval times thought the Earth was flat is a canard: historians know that educated people understood Earth was round even in ancient times. Or consider the claim that humans only use 10% of their brains. This canard has been repeated in movies and books for decades, but brain scans show we use all parts of our brain, just not all at once.

Canards often sound believable at first, which is why they spread. Someone might repeat a canard about a historical figure without realizing it's false, then others repeat it, and soon everyone “knows” something that never actually happened. Unlike an honest mistake, a canard typically starts as a deliberate fabrication. The tricky part about canards is that even after they're proven false, they can keep circulating because they're more entertaining or convenient than the truth.