re ‧ doubt ‧ a ‧ ble /rɪˈdaʊtəb ə l/ BrE AmE adjective literary
[ Date: 1300-1400 ; Language: French ; Origin: redouter 'to fear' , from douter 'to doubt' ]
someone who is redoubtable is a person you respect or fear:
He had never met a more redoubtable fighter.