The Collaborative International Dictionary of English v.0.48:

Terrible \Ter"ri*ble\, a. [F., fr. L. terribilis, fr. terrere to
   frighten. See Terror.]
   1. Adapted or likely to excite terror, awe, or dread;
      dreadful; formidable.
      [1913 Webster]

            Prudent in peace, and terrible in war. --Prior.
      [1913 Webster]

            Thou shalt not be affrighted at them; for the Lord
            thy God is among you, a mighty God and terrible.
                                                  --Deut. vii.
                                                  21.
      [1913 Webster]

   2. Excessive; extreme; severe. [Colloq.]
      [1913 Webster]

            The terrible coldness of the season.  --Clarendon.
      [1913 Webster]

   Syn: Terrific; fearful; frightful; formidable; dreadful;
        horrible; shocking; awful.
        [1913 Webster] -- Ter"ri*ble*ness, n. -- Ter"ri*bly,
        adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

terribly
    adv 1: used as intensifiers; "terribly interesting"; "I'm awful
           sorry" [syn: terribly, awfully, awful,
           frightfully]
    2: in a terrible manner; "she sings terribly" [syn: terribly,
       atrociously, awfully, abominably, abysmally,
       rottenly]

Moby Thesaurus II by Grady Ward, 1.0:

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