The word leech in reference to parasites was first used around 900 CE with the spelling lyce (later forms included laece, liche, leche, leach, and more). It's often thought to come from another Old English word spelled leech of about the same age and with the definition "doctor". However, the Oxford English Dictionary suggests that the latter only influenced the spelling of the former through folk etymology, and that the name for the worm actually comes from the Midde Dutch lake, also meaning "leech". That should actually be a cognate of the English word lake, as both come from the Proto-Germanic root lako, which described bodies of water. If that's true, it ultimately all derives from the Proto-Indo-European reconstruction leg, which meant "leak" or "drain".
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.