illicit has 4 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is licit.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
contrary to accepted morality (especially sexual morality) or convention
“an illicit association with his secretary”
contrary to or forbidden by law
“an illegitimate seizure of power”
Synonyms, antonyms and the broader and narrower relations for illicit come from Princeton University’s WordNet. Full sources and licences.