The word incest was first used in Middle English in an early thirteenth-century manual on how to be a good anchoress. Possibly through Old French inceste, it came from the Latin adjective incestus, which could refer to anything unclean, unholy, or unlawful. It was composed of the prefix in-, meaning "not", and castus, meaning "pure" or "chaste". In fact, it's also where we get our word chaste from, as well as the verb castigate (originally "to. make pure") and the noun caste (associated with "pure" racial groups). Going back further, in- is from Proto-Indo-European ne, meaning "not", and castus comes from the verb carere, meaning "to be cut off from" (as in something pure is "cut off" from all the impure stuff), which in turn gets reconstructed back to the Proto-Indo-European root kes, also meaning "to cut".
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.