Illicit describes something the law, the rules, or society says you should not do. Illicit drugs, illicit trade, an illicit love affair: the word covers both against the law and against what people around you will accept. Though actually, it is not a perfect twin of illegal. Illegal is the courtroom word: a statute forbids it. Illicit is wider and a little softer around the edges — a school rule, a religious rule, or a strong social we do not do that can make something illicit even when no police officer would come. It is a formal, slightly newspaper-like adjective. In speech, people often say illegal, forbidden, or under the table instead. Use illicit when the flavour is hidden, not allowed, and a bit shameful, not when you only mean this is a crime in the code. There is also a rare noun use, but learners almost always meet the adjective.