illicitus

Illicitus, illicita, illicitum, first and second declension: the negative prefix in- on licitus, the participle of licet, "it is permitted". The double l is that prefix assimilating to the following consonant, exactly as in immortalis and irrationalis, so do not look for a root beginning illic-.

It is late Latin rather than classical. Cicero would write vetitus, or nefas, or simply contra legem. In canon law and the penitentials the neuter plural illicita stands on its own as a noun, "unlawful things", which is how it most often appears in this corpus. Keep it apart from illicio, "to entice", which is in- plus lacio and gives the quite different illecebra, "an allurement".