The Collaborative International Dictionary of English v.0.48:

Appalling \Ap*pall"ing\, a.
   Such as to appall; as, an appalling accident. --
   Ap*pall"ing*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Appall \Ap*pall"\, v. t. [imp. & p. p. Appalled; p. pr. & vb.
   n. Appalling.] [OF. appalir to grow pale, make pale; a (L.
   ad) + p[^a]lir to grow pale, to make pale, p[^a]le pale. See
   Pale, a., and cf. Pall.]
   1. To make pale; to blanch. [Obs.]
      [1913 Webster]

            The answer that ye made to me, my dear, . . .
            Hath so appalled my countenance.      --Wyatt.
      [1913 Webster]

   2. To weaken; to enfeeble; to reduce; as, an old appalled
      wight. [Obs.] --Chaucer.
      [1913 Webster]

            Wine, of its own nature, will not congeal and
            freeze, only it will lose the strength, and become
            appalled in extremity of cold.        --Holland.
      [1913 Webster]

   3. To depress or discourage with fear; to impress with fear
      in such a manner that the mind shrinks, or loses its
      firmness; to overcome with sudden terror or horror; to
      dismay; as, the sight appalled the stoutest heart.
      [1913 Webster]

            The house of peers was somewhat appalled at this
            alarum.                               --Clarendon.
      [1913 Webster]

   Syn: To dismay; terrify; daunt; frighten; affright; scare;
        depress. See Dismay.
        [1913 Webster]

WordNet (r) 3.0 (2006):

appalling
    adj 1: causing consternation; "appalling conditions" [syn:
           appalling, dismaying]
    n 1: an experience that appalls; "is it better to view the
         appalling or merely hear of it?"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "appalling":
   astonishing, astounding, atrocious, awe-inspiring, awesome, awful,
   baneful, beastly, bewildering, confounding, conspicuous, daunting,
   dire, direful, dismaying, dread, dreaded, dreadful, dumbfounding,
   egregious, exceptional, extraordinary, fabulous, fantastic, fell,
   formidable, frightful, ghastly, ghoulish, grim, grisly, gruesome,
   hideous, horrendous, horrible, horrid, horrific, horrifying,
   incredible, macabre, marked, marvelous, morbid, notable,
   noteworthy, noticeable, of mark, outstanding, redoubtable,
   remarkable, rotten, schrecklich, shocking, signal, striking,
   superior, terrible, terrific, tragic, tremendous, uncommon,
   unspeakable, wonderful