The word larceny was borrowed in 1475 from the Anglo-Norman word larcen, and where the -y came from is a spot of confusion for linguists. It could have gotten attached to a noun-forming suffix, like we see in the words sympathy or victory, but it also might have been modelled on other crime words like burglary and felony. Either way, larcen comes from Old French larcin and Latin latrocinium, which both also meant "robbery". Latrocinium is a second-declension noun formed from the third-declension word latro, meaning "mercenary" or "highwayman". That comes from Ancient Greek latron, "pay" or "hire" (the connection was the action of hiring mercenaries, but it's sort of funny to me that this is the opposite of the modern definition). Finally, latron derives from Proto-Indo-European leht, meaning "to grant" or "possess".
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.