A canard is "a false or ungrounded report."
Canard refers to a duck in French (as in French dishes like confit de canard), via the Old French quanart, meaning "drake." The connection to rumor and untruth follows a route of creative phrasing. A 16th-century French idiom was vendre des canards à moitié—literally, "to half-sell ducks," but used as a colorful way of saying "to fool" or "to cheat." No one really knows how one half-sells a duck, or where the idiom originated, but the expression was perpetuated enough for canard to carry the meaning of something commonly accepted as true that is actually unfounded:
Although Cheever published 121 stories in the magazine during his lifetime, it is worth remembering that The New Yorker was long regarded as a compromisingly middlebrow place to publish, and the insecure Cheever was always vulnerable to the canard that serious fiction only came in the form of a novel.
—Jonathan Dee, Harper's, April 2009