Source : Webster's Revised Unabridged Dictionary (1913)

Terrifically \Ter*rif"ic*al*ly\, adv.
   In a terrific manner.

Source : WordNet®

terrifically
     adv : (used as an intensifier) extremely well; "her voice is
           superbly disciplined"; "the colors changed wondrously
           slowly" [syn: {wonderfully}, {wondrous}, {wondrously},
           {superbly}, {toppingly}, {marvellously}, {marvelously}]