doubt: [13] English acquired the verb doubt from Old French doter or duter, a descendant of Latin dubitāre ‘waver, be uncertain’ (the b was reintroduced from the Latin spelling in the 15th century). Dubitāre was closely related to Latin dubius ‘uncertain’ (ultimate source of English dubious [16]), which appears to have been based on duo ‘two’, and thus to have meant originally ‘wavering between two possibilities’. In Old French, the sense ‘fear’ developed, and this was an important meaning of the word in Middle English; it survives in the derivative redoubtable [14], literally ‘fearable’. => dubious, redoubtable
