Comment by wmorgan
6 hours ago
Illicit means maybe against the law but definitely against the rules, for example an illicit affair. The word for against the law is illegal, from Latin, or unlawful, from Germanic. I guess the Germanic cousin of "illicit" would be "forbidden."
Extramarital affairs are against the law in many countries and 17 US states. “Illicit affair” is potentially a holdover from when it was illegal more places, not just a conflating of against the rules with illegality.
https://en.wikipedia.org/wiki/Adultery_laws