The Collaborative International Dictionary of English v.0.48:

Terrify \Ter"ri*fy\, v. t. [imp. & p. p. Terrified; p. pr. &
   vb. n. Terrifying.] [L. terrere to frighten + -fy: cf. F.
   terrifier, L. terrificare. See Terrific, and -fy.]
   1. To make terrible. [Obs.]
      [1913 Webster]

            If the law, instead of aggravating and terrifying
            sin, shall give out license, it foils itself.
                                                  --Milton.
      [1913 Webster]

   2. To alarm or shock with fear; to frighten.
      [1913 Webster]

            When ye shall hear of wars . . . be not terrified.
                                                  --Luke xxi. 9.
      [1913 Webster]

WordNet (r) 3.0 (2006):

terrifying
    adj 1: causing extreme terror; "a terrifying wail" [syn:
           terrific, terrifying]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "terrifying":
   alarming, bloodcurdling, frightening, grim, grisly, gruesome,
   hair-raising, hideous, horrible, horrid, horrifying, macabre,
   paralyzing, petrifying, scary, shocking, stunning, stupefying,
   terrible, terror, terror-bearing, terror-breathing,
   terror-breeding, terror-bringing, terror-fraught, terror-giving,
   terror-inspiring, terror-striking, terrorful, terroristic