The word fallacy was first used in a 1481 collection of fables about an anthropomorphic fox. At the very beginning, it was spelled falacye, and then fallacie was the popular way of writing it until fallacy was popularized in the seventeenth century. Through Old English fallace, it all traces to Latin fallax, which meant "deception" or "deceit". That's a noun created from the verb fallere, "to deceive". Finally, Dutch Indo-Europeanist Michiel de Vaan reconstructed it all to Proto-Indo-European sghel, meaning "stumble", although that's uncertain. Semantically, this means that "stumble" took on a metaphorical implication of "causing someone to stumble mentally", and then that transitioned from to a more passive "poor argument" meaning. Usage of fallacy has been decreasing since a peak in the mid-nineteenth century.
Hello! I'm Adam Aleksic, a rising junior studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy trivia, politics, vexillology, geography, board games, conlanging, art history, and law.