Source : Webster's Revised Unabridged Dictionary (1913)

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}.]