redoubtable

The adjective is derived from Late Middle English redoutable (“worthy of honour, venerable; frightening, terrible”), borrowed from Anglo-Norman redoutable and Middle French redoutable, redoubtable, from Old French redotable (modern French redoutable), from redoter (“to fear”) (whence Middle French redoubter, redouter, French redouter) + -able (suffix meaning ‘deserving of, worthy of’). Redoter is derived from re- (intensifying prefix) + doter (“to doubt; to fear”) (from Latin dubitō (“to doubt, be uncertain, waver in opinion”), probably ultimately from Proto-Indo-European dwi- (“apart, asunder; two”) + bʰuH- (“to appear; to be; to become; to grow”), in the sense of being in two minds). The noun is derived from the adjective.