If something looks or sounds ominous, be careful: a threat or an unpleasant event is at hand. If you see an ominous frown on your boss's face, you're in trouble!
Ominous, and the related word omen both come from the 16th century Latin word ōmen "foreboding." However, unlike omen, which is a sign of something good or bad to come, ominous always describes something unpleasant or threatening. If the future looks ominous, you've got nothing to look forward to. Figuratively, an ominous sky or ominous clouds promise rain or a storm.