The Collaborative International Dictionary of English v.0.48:

Tremendous \Tre*men"dous\, a. [L. tremendus that is to be
   trembled at, fearful, fr. tremere to tremble. See Tremble.]
   Fitted to excite fear or terror; such as may astonish or
   terrify by its magnitude, force, or violence; terrible;
   dreadful; as, a tremendous wind; a tremendous shower; a
   tremendous shock or fall.
   [1913 Webster]

         A tremendous mischief was a foot.        --Motley.
   [1913 Webster]

   Syn: Terrible; dreadful; frightful; terrific; horrible;
        awful.
        [1913 Webster] -- Tre*men"dous*ly, adv. --
        Tre*men"dous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

tremendously
    adv 1: extremely; "he was enormously popular" [syn:
           enormously, tremendously, hugely, staggeringly]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "tremendously":
   alarmingly, appallingly, astoundingly, awfully, colossally,
   disconcertingly, dismayingly, divinely, dreadfully, enormously,
   exquisitely, fearfully, frightfully, gigantically, gloriously,
   horrendously, horribly, horridly, horrifically, horrifyingly,
   hugely, immensely, magnificently, marvelously, prodigiously,
   shockingly, startlingly, stupendously, superbly, terribly,
   terrifically, titanically, vastly, wonderfully