The Collaborative International Dictionary of English v.0.48:

Immoral \Im*mor"al\, a. [Pref. im- not + moral: cf. F. immoral.]
   Not moral; inconsistent with rectitude, purity, or good
   morals; contrary to conscience or the divine law; wicked;
   unjust; dishonest; vicious; licentious; as, an immoral man;
   an immoral deed.

   Syn: Wicked; sinful; criminal; vicious; unjust; dishonest;
        depraved; impure; unchaste; profligate; dissolute;
        abandoned; licentious; lewd; obscene.
        [1913 Webster]

WordNet (r) 3.0 (2006):

immoral
    adj 1: deliberately violating accepted principles of right and
           wrong [ant: moral]
    2: not adhering to ethical or moral principles; "base and
       unpatriotic motives"; "a base, degrading way of life";
       "cheating is dishonorable"; "they considered colonialism
       immoral"; "unethical practices in handling public funds"
       [syn: base, immoral]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "immoral":
   abandoned, amoral, bad, base, carnal, concupiscent, conscienceless,
   corrupt, corrupted, criminal, crooked, dark, debauched, degenerate,
   depraved, devious, dirty, dishonest, dishonorable, dissolute,
   doubtful, dubious, evasive, evil, felonious, filthy, fishy,
   flagitious, fraudulent, ill-got, ill-gotten, immodest, impure,
   indecent, indirect, iniquitous, insidious, lascivious, lecherous,
   lewd, libertine, libidinous, licentious, lustful, nefarious,
   nonmoral, not kosher, obscene, pornographic, questionable,
   reprobate, rotten, salacious, shady, shameless, shifty, sinful,
   sinister, slippery, smutty, steeped in vice, suspicious,
   treacherous, tricky, unchaste, unclean, uncleanly, unconscienced,
   unconscientious, unconscionable, underhand, underhanded, unethical,
   unmoral, unprincipled, unregenerate, unsavory, unscrupulous,
   unstraightforward, vice-laden, vice-prone, vicious, vile,
   villainous, wanton, wicked, without remorse, without shame,
   wrong



The Devil's Dictionary (1881-1906):

IMMORAL, adj.  Inexpedient.  Whatever in the long run and with regard
to the greater number of instances men find to be generally
inexpedient comes to be considered wrong, wicked, immoral.  If man's
notions of right and wrong have any other basis than this of
expediency; if they originated, or could have originated, in any other
way; if actions have in themselves a moral character apart from, and
nowise dependent on, their consequences -- then all philosophy is a
lie and reason a disorder of the mind.