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]