sinister in WordNet English dictionary

1. threatening or foreshadowing evil or tragic developments
   "a baleful look"; "forbidding thunderclouds"; "his tone became menacing"; "ominous rumblings of discontent"; "sinister storm clouds"; "a sinister smile"; "his threatening behavior"; "ugly black clouds"; "the situation became ugly"
   baleful  forbidding  menacing  minacious  minatory  ominous  sinister  threatening 
   
2. stemming from evil characteristics or forces; wicked or dishonorable
   "black deeds"; "a black lie"; "his black heart has concocted yet another black deed"; "Darth Vader of the dark side"; "a dark purpose"; "dark undercurrents of ethnic hostility"; "the scheme of some sinister intelligence bent on punishing him"-Thomas Hardy
   black  dark  sinister 
   
3. on or starting from the wearer's left
   "bar sinister"
   sinister