The Collaborative International Dictionary of English v.0.48:

Frightfully \Fright"ful*ly\, adv.
   In a frightful manner; to a frightful dagree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frightfully
    adv 1: used as intensifiers; "terribly interesting"; "I'm awful
           sorry" [syn: terribly, awfully, awful,
           frightfully]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "frightfully":
   abominably, agonizingly, alarmingly, amazingly, appallingly,
   astoundingly, awfully, baldly, balefully, bitterly, blatantly,
   brashly, confoundedly, cruelly, damnably, deadly, deathly,
   deucedly, disconcertingly, dismayingly, distressingly, dolorously,
   dreadfully, egregiously, excessively, excruciatingly, exorbitantly,
   extravagantly, extremely, fearfully, flagrantly, forbiddingly,
   ghastly, grievously, gruesomely, hellishly, hideously,
   horrendously, horribly, horridly, horrifically, horrifyingly,
   improperly, inexcusably, infernally, inordinately, intolerably,
   lamentably, loathsomely, miserably, nakedly, offensively, openly,
   painfully, piteously, repugnantly, repulsively, revoltingly, sadly,
   shatteringly, shockingly, something awful, something fierce,
   sorely, staggeringly, startlingly, surprisingly, terribly,
   terrifically, torturously, tremendously, unashamedly, unbearably,
   unconscionably, unduly, unpardonably, very, woefully