Of or pertaining to an omen or to omens; being or exhibiting an omen; significant; portentous; -- formerly used both in a favorable and unfavorable sense; now chiefly in the latter; foreboding or foreshowing evil; inauspicious; as, an ominous dread.
source: Webster 1913
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.
source: WordNet 3.0
presaging ill fortune
Ill omens. Ill predictions. My words with inauspicious thunderings shook heaven. A dead and ominous silence prevailed. A by-election at a time highly unpropitious for the Government.