[1] : The Collaborative International Dictionary of English v.0.48

ominous \om"inous\, a. l. ominosus, fr. omen. see omen.
   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.
   1913 webster

         he had a good ominous name to have made a peace.
                                                  --bacon.
   1913 webster

         in the heathen worship of god, a sacrifice without a
         heart was accounted ominous.             --south.
   1913 webster -- om"inously, adv. -- om"inousness,
   n.
   1913 webster
see also:
omen om"inously om"inousness 

[2] : WordNet (r) 2.0

ominous
     adj 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" syn: baleful, forbidding,
            menacing, minacious, minatory, sinister, threatening,
             ugly
     2: 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" syn: ill, inauspicious
see also:
baleful forbidding menacing minacious minatory sinister 
threatening ugly ill inauspicious 

[3] : Moby Thesaurus II by Grady Ward, 1.0

131 moby thesaurus words for "ominous":
   abusive, admonitory, apocalyptic, augural, bad, badly off, baleful,
   baneful, black, bludgeoning, blustering, bodeful, boding,
   browbeating, bulldozing, bullying, cautionary, comminatory,
   corroding, corrosive, corrupting, corruptive, counterproductive,
   damaging, dark, deadly, deleterious, denunciatory, depressed,
   detrimental, dire, direful, disadvantageous, disserviceable,
   distressing, donsie, doomful, dreary, evil, evil-starred, fatal,
   fateful, fear-inspiring, forbidding, foreboding, foreshadowing,
   foretelling, foretokening, fortuneless, funest, gloomy, grim,
   hapless, harmful, hectoring, hostile, hurtful, ill, ill off,
   ill-boding, ill-fated, ill-omened, ill-starred, imminent,
   in adverse circumstances, inauspicious, indicative, inhospitable,
   injurious, intimidating, lethal, lowering, luckless, malefic,
   maleficent, malevolent, malign, malignant, mantic, meaningful,
   menacing, minacious, minatory, mischievous, noisome, noxious,
   of evil portent, oracular, out of luck, pernicious, planet-struck,
   poisonous, portending, portentous, predictive, prejudicial,
   premonitory, prognostic, prophetic, sad, scatheful, short of luck,
   sibyllic, sinister, somber, star-crossed, terroristic, terrorizing,
   threatening, threatful, toxic, unblessed, underprivileged,
   unfavorable, unfortunate, unfriendly, unhappy, unlucky,
   unpromising, unpropitious, unprosperous, unprovidential, untoward,
   vaticinal, venenate, veneniferous, venenous, venomous, vicious,
   virulent, warning

Ominous >> O Words
Ominous, definition of term: Ominous
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