aghast (adj.)

c. 1300, agast, "terrified, suddenly filled with frightened amazement," past participle of Middle English agasten "to frighten" (c. 1200), from a- intensive prefix (see a- (1)) + Old English gæstan "to terrify," from gæst "spirit, ghost" (see ghost (n.)). The unetymological -gh- is perhaps a Flemish influence, or after ghost, etc. It became general after 1700.