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):

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, illicit, outlaw(a),
       outlawed, unlawful]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "illicit":
   actionable, adulterous, against the law, anarchic, anarchistic,
   anomic, back-door, banned, barred, black-market, bootleg,
   chargeable, clandestine, contraband, contrary to law, criminal,
   extramarital, felonious, flawed, forbade, forbid, forbidden,
   freeloving, furtive, illegal, illegitimate, impermissible,
   improper, incestuous, irregular, justiciable, lawless,
   nonconstitutional, nonlegal, nonlicit, nonpermissible,
   not permitted, off limits, out of bounds, outlaw, outlawed,
   prohibited, punishable, ruled out, secret, sneaky, taboo, tabooed,
   triable, unallowed, unauthorized, unconstitutional, under the ban,
   under-the-counter, under-the-table, underhanded, unlawful,
   unlicensed, unofficial, unpermissible, unsanctioned, unstatutory,
   untouchable, unwarrantable, unwarranted, verboten, vetoed, wrong,
   wrongful



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ILLICIT. What is unlawful what is forbidden by the law. Vide Unlawful.
     2. This word is frequently used in policies of insurance, where the
assured warrants against illicit trade. By illicit trade is understood that
"which is made unlawful by the laws of the country to which the object is
bound." The assured having entered into this warranty, is required to do no
act which will expose the vessel to be legally condemned. 2 L. R. 337, 338.
Vide Insurance; Trade; Warranty.