Illicit means not allowed by law or custom. It's illegal to kill endangered elephants, but many poachers still engage in the Illicit ivory trade.
Illicit usually refers to something that is not morally proper or acceptable. Illicit activities — like smuggling or counterfeiting — happen under the radar of the law. Illicit is from Latin illicitus, from the prefix in-, "not," plus licitus, "lawful."