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1. Openly distrustful and unwilling to confide
   "He was suspicious of how the difficult situation would work out";
   - leery, mistrustful, untrusting, wary
    
2. Of questionable motive; of doubtful legality, value, truth, honesty, etc.
   "suspicious behaviour";
   - fishy [informal], funny, shady [informal], suspect, dodgy [Brit, informal]

See also: distrustful, questionable

Encyclopedia: Suspicious