The word nefarious was borrowed into the English language at the turn of the seventeenth century directly from the Latin word nefarius, which had a similar meaning of "wicked". That's a modified form of nefas, which could mean "crime" or "impiety". Nefas is composed of ne, meaning "not", and fas, meaning "divine law" (since a crime contradicts divine law). Therefore, something that is nefarious. Ne is an element present in many languages (for example, in Serbian, the word for "no" is ne), ultimately from a Proto-Indo-European reconstruction with the same spelling and definition. Fas derives from PIE behos, which meant "utterance", and that can be simplified further to beh, or "speak". So, if we go back as far as possible, nefarious means "no speech", and calling someone nefarious is tantamount to telling them to shut up.
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.