The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         One illicit . . . transaction always leads to another.
                                                  --Burke.
   -- Il*lic"it*ly, adv. -- Il*lic"it*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illicitness
    n 1: the quality of not conforming strictly to law [ant:
         licitness]