illicit

ih-LIS-it

meaning in plain English

Not allowed by law or rules, kept secret because it is forbidden.

Think illegal plus secret — an illicit deal happens where no one can see.

for example

They were caught selling illicit copies of the movie from a van.

If something is illicit, it is against the rules. Why meet in a dark parking lot? Because the deal is illicit and they do not want to get caught.