Source : Webster's Revised Unabridged Dictionary (1913)

Hideous \Hid"e*ous\ (?; 277), a. [OE. hidous, OF. hidous, hidos,
   hidus, hisdos, hisdous, F. hideux: cf. OF. hide, hisde,
   fright; of uncertain origin; cf. OHG. egid[=i] horror, or L.
   hispidosus, for hispidus rough, bristly, E. hispid.]
   1. Frightful, shocking, or offensive to the eyes; dreadful to
      behold; as, a hideous monster; hideous looks. ``A piteous
      and hideous spectacle.'' --Macaulay.

   2. Distressing or offensive to the ear; exciting terror or
      dismay; as, a hideous noise. ``Hideous cries.'' --Shak.

   3. Hateful; shocking. ``Sure, you have some hideous matter to
      deliver.'' --Shak.

   Syn: Frightful; ghastly; grim; grisly; horrid; dreadful;
        terrible. -- {Hid"e*ous*ly}, adv. -- {Hid"e*ous*ness},
        n.

Source : WordNet®

hideous
     adj 1: grossly offensive to decency or morality; causing horror;
            "subjected to outrageous cruelty"; "a hideous pattern
            of injustice"; "horrific conditions in the mining
            industry" [syn: {horrid}, {horrific}, {outrageous}]
     2: so extremely ugly as to be terrifying; "a hideous scar"; "a
        repulsive mask" [syn: {repulsive}]