redoubtable
Concise Oxford English Dictionary © 2008 Oxford University Press:
redoubtable /rɪˈdaʊtəb(ə)l/
▶adjective often humorous (of a person) formidable, especially as an opponent.
– derivatives
redoubtably adverb.
redoubtably adverb.
– origin ME: from OFr. redoutable, from redouter ‘to fear’, from re- (expressing intensive force) + douter ‘to doubt’.
'redoubtable' also found in these Oxford entries:

