WordWeb Online

1. Threatening or foreshadowing evil or tragic developments
   "sinister storm clouds"; "a sinister smile"
   - baleful, forbidding, menacing, minacious, minatory, ominous, threatening
    
2. Stemming from evil characteristics or forces; wicked or dishonourable
   "the scheme of some sinister intelligence bent on punishing him"
   - black, dark
    
3. On or starting from the wearer's left
   "bar sinister"

See also: alarming, evil, sinistral

Encyclopedia: Sinister