Illicit (adjective)

1. Contrary to or forbidden by law, rules, or custom.
2. Not morally or legally acceptable.

Origin:

Late 16th century: from latin illicitus, from in- "not" + licitus "permitted".

Examples:

1. The police discovered an illicit drug operation.
2. The company was involved in illicit activities.
3. The couple was arrested for illicit behavior.
4. The business was shut down for its illicit practices.
5. The politician was accused of illicit campaign contributions.