nefarious

Where it comes from

Nefarious means extremely wicked or criminal, and it often adds theatrical force to villains and schemes. The adjective comes from Latin nefarius, based on nefas: something contrary to divine law or what was religiously permissible. English therefore inherited a judgment stronger than mere rule-breaking. A secret plan may be clever, dubious, or dishonest; it becomes nefarious when the speaker presents it as deeply immoral. The word turns an ordinary bad act into a violation of the deepest rules.

How the meaning developed

Use nefarious when the point is not secrecy but outrageous wickedness.

In use

Instead of “That looks shady.”
Try “Something nefarious is afoot.”

“Nefarious” means extremely wicked. “Afoot” means happening or in progress.
The rewrite is deliberately dramatic, not a neutral description.