The Collaborative International Dictionary of English v.0.48:

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.
   [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"i*nous*ly, adv. -- Om"i*nous*ness,
   n.
   [1913 Webster]