Nefarious means evil or criminal, and not in a small, everyday way. You use it for plans, deeds, and people that feel wicked — stealing, cheating on a large scale, dark schemes. It is a formal, slightly dramatic word. News writers and storytellers like it; friends almost never say it about a classmate who was rude. Though actually, English speakers often use it with a little smile, as if they are putting on a villain's cape: 'his nefarious plan to steal the last biscuit'. The tidy dictionary says 'evil or criminal', and that is true, but in real talk the word is often a bit like acting in a play. It is not a gentle word for 'naughty' or 'a bit wrong'.