Nefarious means wicked, and usually done in secret. Nefarious purposes, a nefarious scheme, nefarious activity.
It is so strong that it is now used mostly with a smile, and nefarious purposes is nearly always half a joke. The Latin behind it means against divine law, which is a great deal heavier than any modern use of the word.
1adjectivenefariousWicked, and usually done in secret.
“The villain in the film has a nefarious plan to poison the city's water.”
nefarious is a quality.
Latin nefarius comes from nefas, something contrary to divine law — ne, not, plus fas, what is right.
Latin
nefārius — execrable, abominable
English, today
nefarious
Built from
From Latin nefārius (“execrable, abominable”), from nefās (“something contrary to divine law, an impious deed, sin, crime”), from ne- (“not”) + fās (“the dictates of religion, divine law”), which is related to Latin for (“to speak, to say”) and cognate to Ancient Greek φημί (“to say”).
Etymology from Wiktionary, CC BY-SA