The Collaborative International Dictionary of English v.0.48:

Horrible \Hor"ri*ble\, a. [OE. horrible, orrible, OF. horrible,
   orrible, F. horrible, fr. L. horribilis, fr. horrere. See
   Horror.]
   Exciting, or tending to excite, horror or fear; dreadful;
   terrible; shocking; hideous; as, a horrible sight; a horrible
   story; a horrible murder.
   [1913 Webster]

         A dungeon horrible on all sides round.   --Milton.

   Syn: Dreadful; frightful; fearful; terrible; awful; terrific;
        shocking; hideous; horrid.
        [1913 Webster]

WordNet (r) 3.0 (2006):

horrible
    adj 1: provoking horror; "an atrocious automobile accident"; "a
           frightful crime of decapitation"; "an alarming, even
           horrifying, picture"; "war is beyond all words horrible"-
           Winston Churchill; "an ugly wound" [syn: atrocious,
           frightful, horrifying, horrible, ugly]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "horrible":
   abhorrent, abominable, appalling, arrant, astounding, atrocious,
   awe-inspiring, awesome, awful, baneful, base, beastly,
   beneath contempt, blameworthy, brutal, contemptible, deadly,
   deplorable, despicable, detestable, dire, direful, disagreeable,
   disgusting, dread, dreaded, dreadful, egregious, enormous, fearful,
   fell, fetid, filthy, flagrant, forbidding, formidable, foul,
   frightening, frightful, fulsome, ghastly, ghoulish, grievous, grim,
   grisly, gross, gruesome, harrowing, hateful, heinous, hideous,
   horrendous, horrid, horrific, horrifying, howling, infamous,
   lamentable, loathsome, lousy, lurid, macabre, monstrous, morbid,
   nasty, nauseating, nauseous, nefarious, noisome, notorious,
   obnoxious, odious, offensive, outrageous, pitiable, pitiful, rank,
   redoubtable, regrettable, repellent, repelling, reprehensible,
   repugnant, repulsive, revolting, rotten, rousing, sad, scandalous,
   schlock, schrecklich, scurvy, shabby, shameful, shocking, shoddy,
   sickening, sordid, squalid, terrible, terrific, terrifying,
   thumping, too bad, tragic, tremendous, unclean, unpleasant,
   unspeakable, vile, villainous, whacking, woeful, worst, worthless,
   wretched