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®

illicit
     adj 1: contrary to accepted morality (especially sexual morality)
            or convention; "an illicit association with his
            secretary" [ant: {licit}]
     2: contrary to or forbidden by law; "an illegitimate seizure of
        power"; "illicit trade"; "an outlaw strike"; "unlawful
        measures" [syn: {illegitimate}, {outlaw(a)}, {outlawed}, {unlawful}]