Source : Webster's Revised Unabridged Dictionary (1913)

Ominous \Om"i*nous\, 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.

         He had a good ominous name to have made a peace.
                                                  --Bacon.

         In the heathen worship of God, a sacrifice without a
         heart was accounted ominous.             --South.
   -- {Om"i*nous*ly}, adv. -- {Om"i*nous*ness}, n.

Source : WordNet®

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}]