Poisson d’avril: “April fish” (French): a person who is taken in by an April Fools’ Day prank. By extension, the prank itself.
Holidays that celebrate foolishness and pranking have antecedents in the Roman era and possible earlier. “Foolish” holidays are still observed in many countries around the world. The English custom goes back to the late 17th century; the French tradition appears to predate it by a century or so. One theory, according to the French-language Wikipedia entry, is that until 1564 the calendar year began on April 1; some traditionalists refused to accept the change, and they were mocked by being given fake gifts.
The poisson of poisson d’avril is a paper fish that is stealthily attached to the back of an unsuspecting person. In France, it’s a children’s game, although the trope is often appropriated by the media, as in this photo from 2012:
Former Prime Minister President Nicolas Sarkozy pins a fish on the back of his successor prime minister, François Fillon, while telling him (rough translation): “François, you’ll stay in Matignon [the prime minister’s residence] until 2012, I swear!” Hollande Fillon: “Cool!” (The joke: Sarkozy was defeated in May 2012.)*
Poisson d’Avril is also the name of a 1954 French film directed by Gilles Grangier.
The French newspaper Le Figaro has compiled the best French poissons d’avril of 2013, including “Drones to Distribute Newspapers” and “Sarkozy to Move to Belgium.” For technophile Anglophones, here’s TechCrunch’s annual “Great List of April Foolings.”
My own favorite April 1 post of 2013 is this little masterpiece from Brand New: “Google Chrome Broken Image Icon Heals Broken Hearts.” On the more scholarly side, here’s a Wordnik list of April Fools’ Day words, including dupe, gull, and cat’s-paw.
UPDATE: PayWag, HondaHAIR, Snickettes, and other branding foolery in AdWeek’s “Super Bowl of Satire.”
* One of you will surely be able to improve on my translation. UPDATE: Thanks to Pat in Toulouse! The fault lay not so much in my translation as in my ignorance of French political figures.