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]

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