The Collaborative International Dictionary of English v.0.48:

Unlawful \Un*law"ful\, a.
   Not lawful; contrary to law. -- Un*law"ful*ly, adv. --
   Un*law"ful*ness, n.
   [1913 Webster]

   Unlawful assembly. (Law) See under Assembly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unlawful
    adj 1: not conforming to legality, moral law, or social
           convention; "an unconventional marriage"; "improper
           banking practices" [syn: improper, unconventional,
           unlawful]
    2: contrary to or prohibited by or defiant of law; "unlawful
       measures"; "unlawful money"; "unlawful hunters" [ant:
       lawful]
    3: not morally right or permissible; "unlawful love"
    4: having no legally established claim; "the wrongful heir to
       the throne" [syn: unlawful, wrongful]
    5: 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:

99 Moby Thesaurus words for "unlawful":
   aberrant, abnormal, abominable, actionable, against the law,
   anarchic, anarchistic, anomic, atrocious, banned, barred,
   black-market, bootleg, chargeable, contraband, contrary to law,
   criminal, delinquent, deviant, disgraceful, evil, exceptionable,
   felonious, flawed, forbade, forbid, forbidden, hardly the thing,
   ignominious, illegal, illegitimate, illicit, impermissible,
   improper, inappropriate, incorrect, indecorous, infamous,
   iniquitous, interdicted, intolerable, irregular, justiciable,
   lawless, nefarious, nonconstitutional, nonlegal, nonlicit,
   nonpermissible, not done, not permitted, not the thing,
   objectionable, off limits, off-base, off-color, out of bounds,
   out-of-line, outlaw, outlawed, prohibited, proscribed, punishable,
   ruled out, sacrilegious, scandalous, shameful, shameless, sinful,
   taboo, tabooed, terrible, triable, unallowed, unauthorized,
   unconstitutional, under the ban, under the table,
   under-the-counter, under-the-table, undue, unfit, unfitting,
   unlicensed, unofficial, unpermissible, unrighteous, unsanctioned,
   unseemly, unstatutory, unsuitable, untouchable, unwarrantable,
   unwarranted, verboten, vetoed, wicked, wrong, wrongful



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

UNLAWFUL. That which is contrary to law.
     2. There are two kinds of contracts which are unlawful; those which are
void, and those which are not. When the law expressly prohibits the
transaction in respect of which the agreement is entered into and declares
it to be void, it is absolutely so. 3 Binn. R. 533. But when it is merely
prohibited, without being made void, although unlawful, it is not void. 12
Serg. & Rawle, 237; Chitty, Contr. 230; 23 Amer. Jur. 1 to 23; 1 Mod. 35; 8
East, R. 236, 237; 3 Taunt. R. 244; Hob. 14. Vide Condition; Void.