The Collaborative International Dictionary of English v.0.48:

Redoubtable \Re*doubt"a*ble\ (-?*b'l), a. [F. redoutable,
   formerly also spelt redoubtable.]
   Formidable; dread; terrible to foes; as, a redoubtable hero;
   hence, valiant; -- often in contempt or burlesque. [Written
   also redoutable.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

redoubtable
    adj 1: 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]
    2: worthy of respect or honor; "born of a redoubtable family"

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "redoubtable":
   appalling, astounding, awe-inspiring, awesome, awful, celebrated,
   dire, direful, distinguished, dread, dreaded, dreadful, eminent,
   famed, fell, formidable, frightful, ghastly, ghoulish, great, grim,
   grisly, gruesome, hideous, horrendous, horrible, horrid, horrific,
   horrifying, illustrious, macabre, morbid, notable, prominent,
   renowned, schrecklich, shocking, terrible, terrific, tremendous