The Collaborative International Dictionary of English v.0.48:

Iniquitous \In*iq"ui*tous\, a. [From Iniquity.]
   Characterized by iniquity; unjust; wicked; as, an iniquitous
   bargain; an iniquitous proceeding.
   [1913 Webster]

         Demagogues . . . bribed to this iniquitous service.
                                                  --Burke.

   Syn: Wicked; wrong; unjust; unrighteous; nefarious; criminal.

   Usage: Iniquitous, Wicked, Nefarious. Wicked is the
          generic term. Iniquitous is stronger, denoting a
          violation of the rights of others, usually by fraud or
          circumvention. Nefarious is still stronger, implying a
          breach of the most sacred obligations, and points more
          directly to the intrinsic badness of the deed.
          [1913 Webster]

WordNet (r) 3.0 (2006):

iniquitous
    adj 1: characterized by iniquity; wicked because it is believed
           to be a sin; "iniquitous deeds"; "he said it was sinful
           to wear lipstick"; "ungodly acts" [syn: iniquitous,
           sinful, ungodly]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "iniquitous":
   abominable, arrant, atrocious, bad, baleful, base, bitchy, black,
   blamable, blameworthy, criminal, cussed, damnable, dark,
   disgraceful, evil, evildoing, execrable, felonious, flagitious,
   flagrant, foul, harmful, hateful, heinous, illegal, immoral,
   improper, inequitable, infamous, invidious, knavish, low,
   malefactory, malefic, maleficent, malfeasant, malicious, malign,
   malignant, mean, monstrous, nasty, naughty, nefarious, noxious,
   ornery, peccant, rank, reprehensible, reprobate, scandalous,
   shameful, sinful, unbalanced, undeserved, undue, unequal,
   unequitable, uneven, unforgivable, unjust, unlawful, unmeet,
   unmerited, unpardonable, unrightful, unspeakable, unworthy,
   vicious, vile, villainous, wicked, wrong, wrongdoing, wrongful