The Collaborative International Dictionary of English v.0.48:

Hideous \Hid"e*ous\ (h[i^]d"[-e]*[u^]s; 277), a. [OE. hidous,
   OF. hidous, hidos, hidus, hisdos, hisdous, F. hideux: cf. OF.
   hide, hisde, fright; of uncertain origin; cf. OHG. egid[imac]
   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.
      [1913 Webster]

   2. Distressing or offensive to the ear; exciting terror or
      dismay; as, a hideous noise. "Hideous cries." --Shak.
      [1913 Webster]

   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.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hideousness
    n 1: dreadful ugliness; horrible repulsiveness