Source : Webster's Revised Unabridged Dictionary (1913)

Agast \A*gast"\ or Aghast \A*ghast"\, v. t.
   To affright; to terrify. [Obs.] --Chaucer. Spenser.

Aghast \A*ghast"\, a & p. p. [OE. agast, agasted, p. p. of
   agasten to terrify, fr. AS. pref. [=a]- (cf. Goth. us-, G.
   er-, orig. meaning out) + g?stan to terrify, torment: cf.
   Goth. usgaisjan to terrify, primitively to fix, to root to
   the spot with terror; akin to L. haerere to stick fast,
   cling. See {Gaze}, {Hesitate}.]
   Terrified; struck with amazement; showing signs of terror or
   horror.

         Aghast he waked; and, starting from his bed, Cold sweat
         in clammy drops his limbs o'erspread.    --Dryden.

         The commissioners read and stood aghast. --Macaulay.

Aghast \A*ghast"\, v. t.
   See {Agast}, v. t. [Obs.]

Source : WordNet®

aghast
     adj : struck with fear, dread, or consternation [syn: {aghast(p)},
            {appalled}, {dismayed}, {shocked}]