Source : Webster's Revised Unabridged Dictionary (1913)

Illicit \Il*lic"it\, a. [L. illicitus; pref. il- not + licitus,
   p. p. of licere to be allowed or permitted: cf. F. illicite.
   See {In-} not, and {License}.]
   Not permitted or allowed; prohibited; unlawful; as, illicit
   trade; illicit intercourse; illicit pleasure.

         One illicit . . . transaction always leads to another.
                                                  --Burke.
   -- {Il*lic"it*ly}, adv. -- {Il*lic"it*ness}, n.

Source : WordNet®

illicitness
     n : the quality of not conforming strictly to law [ant: {licitness}]