The Collaborative International Dictionary of English v.0.48:

Fishy \Fish"y\, a.
   1. Consisting of fish; fishlike; having the qualities or
      taste of fish; abounding in fish. --Pope.
      [1913 Webster]

   2. Extravagant, like some stories about catching fish;
      improbable; also, rank or foul. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

fishy
    adj 1: of or relating to or resembling fish; "the soup had a
           fishy smell"
    2: not as expected; "there was something fishy about the
       accident"; "up to some funny business"; "some definitely
       queer goings-on"; "a shady deal"; "her motives were suspect";
       "suspicious behavior" [syn: fishy, funny, shady,
       suspect, suspicious]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "fishy":
   ambiguous, amoral, beguiling, blank, catchy, conscienceless,
   corrupt, corrupted, criminal, crooked, dark, deadpan, deceiving,
   deceptive, delusive, delusory, devious, dishonest, dishonorable,
   doubtable, doubtful, dubious, dubitable, eellike, empty, equivocal,
   evasive, expressionless, fallacious, false, felonious, fraudulent,
   funny, glassy, hallucinatory, ill-got, ill-gotten, illusive,
   illusory, immoral, impassive, implausible, improbable, indirect,
   inexpressive, insidious, misleading, not kosher, odd, peculiar,
   piscatorial, piscatory, pisciform, piscine, poker-faced,
   problematic, queer, questionable, rotten, selachian, shady,
   shameless, sharkish, shifty, sinister, slippery, strange, suspect,
   suspicious, trickish, tricksy, tricky, uncertain, unconscienced,
   unconscientious, unconscionable, underhand, underhanded, unethical,
   unexpressive, unlikely, unprincipled, unsavory, unscrupulous,
   unstraightforward, vacant, without remorse, without shame,
   wooden