The Collaborative International Dictionary of English v.0.48:

Suspicious \Sus*pi"cious\, a. [OE. suspecious; cf. L.
   suspiciosus. See Suspicion.]
   1. Inclined to suspect; given or prone to suspicion; apt to
      imagine without proof.
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            Nature itself, after it has done an injury, will
            ever be suspicious; and no man can love the person
            he suspects.                          --South.
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            Many mischievous insects are daily at work to make
            men of merit suspicious of each other. --Pope.
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   2. Indicating suspicion, mistrust, or fear.
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            We have a suspicious, fearful, constrained
            countenance.                          --Swift.
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   3. Liable to suspicion; adapted to raise suspicion; giving
      reason to imagine ill; questionable; as, an author of
      suspicious innovations; suspicious circumstances.
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            I spy a black, suspicious, threatening could.
                                                  --Shak.
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   Syn: Jealous; distrustful; mistrustful; doubtful;
        questionable. See Jealous.
        [1913 Webster] -- Sus*pi"cious*ly, adv. --
        Sus*pi"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

suspicious
    adj 1: openly distrustful and unwilling to confide [syn:
           leery, mistrustful, suspicious, untrusting,
           wary]
    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:

153 Moby Thesaurus words for "suspicious":
   Pyrrhonic, a bit thick, a bit thin, absurd, agnostic, amoral,
   apprehensive, arguable, at issue, beyond belief, borderline, cagey,
   careful, cautious, chary, confutable, conjectural, conscienceless,
   contestable, controversial, controvertible, corrupt, corrupted,
   criminal, crooked, dark, debatable, deniable, devious,
   disbelieving, dishonest, dishonorable, disinclined to believe,
   disposed to doubt, disputable, distrustful, distrustfully,
   doubtable, doubtful, doubtfully, doubting, dubious, dubitable,
   envious, evasive, felonious, fishy, fraudulent, from Missouri,
   green, green with jealousy, green-eyed, guarded, hard of belief,
   hard to believe, horn-mad, iffy, ill-got, ill-gotten, immoral,
   impervious to persuasion, implausible, in dispute, in doubt,
   in dubio, in question, inconceivable, inconvincible, incredible,
   incredulous, indirect, insidious, invidious, jaundice-eyed,
   jaundiced, jealous, leery, mistakable, mistrustful, mistrusting,
   moot, not deserving belief, not kosher, open, open to doubt,
   open to question, open to suspicion, passing belief, preposterous,
   problematic, queer, questionable, questioning, refutable,
   ridiculous, rotten, sceptical, scrupulous, shady, shaky, shameless,
   shifty, shy, shy of belief, sinister, skeptical, skeptically,
   slippery, speculative, staggering belief, suppositional, suspect,
   suspected, suspecting, suspiciously, tall, thick, thin, tricky,
   unbelievable, unbelieving, uncertain, unconscienced,
   unconscientious, unconscionable, unconvincible, unconvincing,
   uncredulous, under suspicion, underhand, underhanded, unearthly,
   uneasy, unethical, ungodly, unimaginable, unpersuadable,
   unpersuasible, unprincipled, unsavory, unscrupulous,
   unstraightforward, unsure, unthinkable, untrusting,
   unwilling to accept, unworthy of belief, wary, watchful,
   without remorse, without shame, yellow, yellow-eyed