1) Adulterous 2) Backdoor 3) Extracurricular 4) Extramarital 5) Forbidden 6) Forbidden by law 7) French word used in English 8) Like gunrunning 9) Like hanky-panky 10) Like racketeering 11) Like some stills 12) Not legal 13) Not permitted 14) Proscribed 15) Unauthorized 16) Unlawful 17) Unlawful forbidden 1) Adulterous 2) Dirty 3) Extramarital 4) Forbidden 5) Illegal 6) Illegitimate 7) Outlaw 8) Outlawed 9) Prohibited 10) Unlawful 
• (a.) Not permitted or allowed; prohibited; unlawful; as, illicit trade; illicit intercourse; illicit pleasure.
[Adjective] Against the law or not accepted by a society.
Refer to the definition for illegal.
Il·lic'it adjective [ Latin illicitus ; prefix il- not + licitus , past participle of licere to be allowed or permitted: confer French illicite . See In- not, and License .] Not permitted or allowed; prohibited; unlawful; as, illicit trade; 
unlawful forbidden 
[adj] - contrary to accepted morality (especially sexual morality) or convention
illicit 1. Not allowed by the law. 2. Considered wrong or unacceptable by prevailing social customs or standards. 3. Disapproved of or not permitted for moral or ethical reasons.
adjective contrary to accepted morality (especially sexual morality) or convention; `an illicit association with his secretary`
not legally permitted or authorized; unlicensed; unlawful. · disapproved of or not permitted for moral or ethical reasons.
contrary to accepted morality or convention[Wise words] contrary to or forbidden by law