In Practical, Standard English, consider ‘Licit’
[Latin, licitus, past participle of licere ‘it is allowed’]
Now, Licit is often misused and confused with Lawful; and they are close synonyms. Consider:
[Latin, licet, it is permitted]
Why do we say activities are illicit?
We usually say what we mean and not the opposite.
LICIT: not forbidden by law. It applies to strict conformity to the provisions of law. And that is especially what the law regulates conforming to the requirements of the law.
LAWFUL: permissible; permitted by law; legitimate; constituted by law; valid or regarded as valid [of marriage born of lawful marriage; said of offspring] Having full, legal rights (synonyms include: just, right)
But, when it comes to be law-abiding, Licit becomes clunky.
Most things would, ostensibly, be lawful.
It becomes more convenient to say what is and what is not Illicit.
Illicit is now preferred to Licit in common, American usage.