Definitions for ominousˈɒm ə nəs

Random House Webster's College Dictionary

om•i•nous^*ˈɒm ə nəs(adj.)

portending evil or harm; foreboding; threatening; inauspicious.
having the significance of an omen.

^* Syn: ominous , portentous , fateful , threatening describe something that foretells a serious and significant outcome or consequence. ominous suggests an evil or harmful consequence: ominous storm clouds.portentous , although it may point to evil or disaster, more often describes something momentous or important: a portentous change in foreign policy.fateful also stresses the great or decisive importance of what it describes: a fateful encounter between two influential leaders.threatening may point to calamity or mere unpleasantness, but usu. suggests that the outcome is imminent: a threatening rumble from a volcano.

Origin of ominous:

1580–90; < L ōminōsus portentous, der. of ōmin-omen

om′i•nous•ness(n.)

Princeton's WordNet

baleful, forbidding, menacing, minacious, minatory, ominous, sinister, threatening(adj)
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"
ill, inauspicious, ominous(adj)
presaging ill fortune
"ill omens"; "ill predictions"; "my words with inauspicious thunderings shook heaven"- P.B.Shelley; "a dead and ominous silence prevailed"; "a by-election at a time highly unpropitious for the Government"

Wiktionary

ominous(Adjective)
Of or pertaining to an omen or to omens; being or exhibiting an omen; significant.
ominous(Adjective)
Specifically, giving indication of a coming ill; being an evil omen; threatening; portentous; inauspicious.

Origin: From ominosus, from omen, from os + -men