The Collaborative International Dictionary of English v.0.48:

Terrifically \Ter*rif"ic*al*ly\, adv.
   In a terrific manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

terrifically
    adv 1: (used as an intensifier) extremely well; "her voice is
           superbly disciplined"; "the colors changed wondrously
           slowly" [syn: wonderfully, wondrous, wondrously,
           superbly, toppingly, marvellously, terrifically,
           marvelously]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "terrifically":
   alarmingly, almighty, appallingly, astoundingly, awfully,
   disconcertingly, dismayingly, divinely, dreadfully, exceedingly,
   exquisitely, fearfully, frightfully, gloriously, horrendously,
   horribly, horridly, horrifically, horrifyingly, immensely, just,
   magnificently, marvelously, mightily, mighty, only too, powerful,
   powerfully, pretty, quite, real, really, right, shockingly, so,
   startlingly, superbly, terribly, tremendously, very, wonderfully