threatening 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. darkened by clouds
   "a heavy sky"
   heavy  lowering  sullen  threatening