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]