April fool (an April Fool's Day joke); April fool (a person subjected to an April Fool's Day joke); paper fish pinned to someone's back as a traditional April Fool's Day joke
“1843, Théodore de Jolimont, "Monologie du mois d'avril", in L'Art en Provence, vol. 7, section IV (unnumbered page). Ces poissons d'avril qui font grincer les dents, maugréer et maudire, et que dame Justice n'épargne guère aux incorrigibles plaideurs;” — These April Fools' Jokes, which make people grind their teeth, grumble and curse, and which Lady Justice hardly spares the incorrigible litigants;
Literally, “April fish”. Compare Italian pesce d'aprile, Portuguese peixe de abril. From the practical joke played on April Fools', of pinning a fish to the back of a person.