The Collaborative International Dictionary of English v.0.48:

Dreadfully \Dread"ful*ly\, adv.
   In a dreadful manner; terribly. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dreadfully
    adv 1: of a dreadful kind; "there was a dreadfully bloody
           accident on the road this morning" [syn: dreadfully,
           awfully, horribly]
    2: in a dreadful manner; "as he looks at the mess he has left
       behind he must wonder how the Brits so often managed to
       succeed in the kind of situation where he has so dismally
       failed" [syn: dismally, dreadfully]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "dreadfully":
   abominably, agonizingly, alarmingly, appallingly, astoundingly,
   atrociously, awful, awfully, baldly, balefully, banefully, basely,
   bestially, big, bitterly, blatantly, brashly, brutally,
   confoundedly, contemptibly, cruelly, damnably, damned, deadly,
   deathly, despicably, detestably, deucedly, direly, disconcertingly,
   disgustingly, dismayingly, distressingly, dolorously, dreadful,
   egregiously, exceedingly, excessively, excruciatingly,
   exorbitantly, extravagantly, extremely, fearfully, flagrantly,
   forbiddingly, foully, frightfully, fulsomely, ghastly, grievously,
   grimly, grossly, gruesomely, hellishly, hideously, horrendously,
   horribly, horridly, horrifically, horrifyingly, improperly,
   inexcusably, infamously, infernally, inordinately, intolerably,
   lamentably, loathsomely, miserably, nakedly, nastily, nauseatingly,
   notoriously, obnoxiously, odiously, offensively, openly,
   outrageously, painfully, piteously, repugnantly, repulsively,
   revoltingly, sadly, savagely, scandalously, shabbily, shamefully,
   shatteringly, shockingly, something awful, something fierce,
   sordidly, sorely, staggeringly, startlingly, strikingly,
   surpassingly, terribly, terrifically, torturously, tragically,
   tremendously, unashamedly, unbearably, unconscionably, unduly,
   unpardonably, viciously, vilely, woefully, wretchedly