What is the difference between “illicit” and “illegal”?
An illegal action only breaks the law. An illicit action breaks ethical or moral standards (and probably the law as well).
contrary to or forbidden by official rules, regulations, etc.
not legally permitted or authorized; unlicensed; unlawful.
disapproved of or not permitted for moral or ethical reasons.
As you see here, illicit can be used to mean illegal, but its second meaning makes all the difference. So if you want to preserve our precious English nuances, choose wisely!