The Collaborative International Dictionary of English v.0.48:

Unrighteous \Un*right"eous\, a. [OE. unrightwise, AS.
   unrihtw[imac]s. See Un- not, and Righteous.]
   [1913 Webster]
   1. Not righteous; evil; wicked; sinful; as, an unrighteous
      man.
      [1913 Webster]

   2. Contrary to law and equity; unjust; as, an unrighteous
      decree or sentence.
      [1913 Webster] -- Un*right"eous*ly, adv. --
      Un*right"eous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unrighteous
    adj 1: not righteous; "an unrighteous man"; "an unrighteous law"
           [ant: righteous]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "unrighteous":
   Adamic, aberrant, abnormal, abominable, atrocious, backsliding,
   carnal, criminal, delinquent, deviant, disgraceful, erring, evil,
   fallen, fleshly, frail, godless, hardly the thing, ignominious,
   illegal, impious, improper, impure, inappropriate, incorrect,
   indecorous, infamous, infirm, irreligious, lapsed, not done,
   not the thing, of easy virtue, off-base, off-color, out-of-line,
   peccable, postlapsarian, prodigal, recidivist, recidivistic,
   sacrilegious, scandalous, shameful, shameless, sinful, terrible,
   un-Christly, unangelic, unchaste, unchristian, unclean, undue,
   unequitable, unfair, unfit, unfitting, ungodly, ungood, unholy,
   unjust, unlawful, unsaintly, unseemly, unsuitable, unvirtuous,
   virtueless, wanton, wayward, weak, wicked, wrong, wrongful