The Collaborative International Dictionary of English v.0.48:

Formidable \For"mi*da*ble\, a. [L. formidabilis, fr. formidare
   to fear, dread: cf. F. formidable.]
   Exciting fear or apprehension; impressing dread; adapted to
   excite fear and deter from approach, encounter, or
   undertaking; alarming.
   [1913 Webster]

         They seemed to fear the formodable sight. --Dryden.
   [1913 Webster]

         I swell my preface into a volume, and make it
         formidable, when you see so many pages behind. --Drydn.

   Syn: Dreadful; fearful; terrible; frightful; shocking;
        horrible; terrific; tremendous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

formidable
    adj 1: extremely impressive in strength or excellence; "a
           formidable opponent"; "the challenge was formidable";
           "had a formidable array of compositions to his credit";
           "the formidable army of brains at the Prime Minister's
           disposal"
    2: inspiring fear; "the formidable prospect of major surgery";
       "a tougher and more redoubtable adversary than the heel-
       clicking, jackbooted fanatic"- G.H.Johnston; "something
       unnerving and prisonlike about high grey wall" [syn:
       formidable, redoubtable, unnerving]

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "formidable":
   Herculean, abstruse, alarming, appalling, arduous, astonishing,
   astounding, awe-inspiring, awesome, awful, brutal, burdensome,
   challenging, complex, conspicuous, critical, daunting, delicate,
   demanding, difficile, difficult, dire, direful, dread, dreaded,
   dreadful, earnest, effortful, egregious, exacting, exceptional,
   extraordinary, fabulous, fantastic, fatal, fateful, fell, freaky,
   frightening, frightful, ghastly, ghoulish, grave, grim, grisly,
   gruesome, hairy, hard, hard-earned, hard-fought, heavy, hideous,
   horrendous, horrible, horrid, horrific, horrifying, imposing,
   impressive, incredible, indomitable, intimidating, intricate,
   jawbreaking, knotted, knotty, labored, laborious, macabre, marked,
   marvelous, mean, menacing, mighty, mind-blowing, mind-boggling,
   morbid, no picnic, not easy, notable, noteworthy, noticeable,
   of mark, onerous, operose, outstanding, overwhelming, petrifying,
   portentous, powerful, prodigious, redoubtable, remarkable,
   rigorous, rough, rude, rugged, schrecklich, serious,
   set with thorns, severe, shocking, signal, sober, solemn, spiny,
   staggering, steep, strenuous, striking, superior, terrible,
   terrific, terrifying, thorny, ticklish, toilsome, tough,
   tremendous, tricky, unbelievable, uncommon, uphill, weighty,
   wicked, wonderful