« Poisson d'avril » au sens moderne (Jean de Labrune)

1691

The earliest known occurrence of 'poisson d'avril' meaning 'tromperie, mystification traditionnelle du 1er avril', the modern April Fools sense, appears in Jean de Labrune's La Vie de Charles V, duc de Lorraine (1691), per the CNRTL/TLFi etymological entry. This marks the semantic shift from the 15th-century 'go-between' sense to the prank sense, roughly two centuries later, and is the key dated milestone for the French custom as such.