WordNet (r) 3.0 (2006):

unethical
    adj 1: not conforming to approved standards of social or
           professional behavior; "unethical business practices"
           [ant: ethical]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "unethical":
   amoral, conscienceless, corrupt, corrupted, criminal, crooked,
   dark, devious, dishonest, dishonorable, doubtful, dubious, evasive,
   felonious, fishy, fraudulent, ill-got, ill-gotten, immoral,
   indirect, insidious, mercenary, not kosher, questionable, rotten,
   shady, shameless, shifty, sinister, slippery, suspicious, tricky,
   unconscienced, unconscientious, unconscionable, underhand,
   underhanded, unprincipled, unsavory, unscrupulous,
   unstraightforward, venal, without remorse, without shame