Q: We were celebrating our 25th anniversary with confit de canard when this question came up: How did the French word for a duck come to mean a false story in English?
A: The short answer is that canard has both senses in French, and English borrowed one of them. But how did the word for a duck come to mean a false story in French?
The Chambers Dictionary of Etymology says “the sense of a false or exaggerated story comes from a French expression of the late 1500s vendre un canard à moitié to half-sell a duck (i.e., not to sell it at all), hence to take in, deceive, make a fool of.”
The “canard” entry in Chambers echoes the work of the 19th-century French lexicographer Émile Littré and the 17th century English lexicographer Randle Cotgrave.
Littré, in the Dictionnaire de la Langue Française (1863–77), has a 1612 citation for the expression bailleur de canards (literally deliverer of ducks), used to mean a teller of absurd stories.
And Cotgrave, in A Dictionarie of the French and English Tongues (1611), defines the French expression vendeur de canards a moitié as “a cousener, guller, cogger; foister, lyer.”
From these beginnings, Chambers says, “The sense of ‘false news spread to deceive the public’ appeared in French in 1750.”
The earliest English example of “canard” in the Oxford English Dictionary is from an 1864 edition of An American Dictionary of the English Language, edited by Chauncey A. Goodrich and Noah Porter.
However, the OED quotes James Murray, the principal editor of its first edition, as saying, “I saw the word in print before 1850.”
Oxford suggests that the false sense of “canard” may have been popularized when it became the subject of a syndicated language column, “The Romance of Words,” that appeared in many American newspapers in the 1920s.
The column traced the phony sense of the French word “to an absurd fabricated story purporting to illustrate the voracity of ducks,” according to the OED.
We won’t go into details here, but the story—or, rather, the canard—concerns a scientist who supposedly got a bunch of ducks to eat each other until only one was left.
“As this account has been widely circulated,” the OED says, “it is possible that it has contributed to render the word more familiar, and thus more used, in English.”
from Grammarphobia http://www.grammarphobia.com/blog/2016/04/canard.html