doubtyoudaoicibaDictYouDict[doubt 词源字典]
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[doubt etymology, doubt origin, 英语词源]
doubt (v.)youdaoicibaDictYouDict
early 13c., "to dread, fear," from Old French doter "doubt, be doubtful; be afraid," from Latin dubitare "to doubt, question, hesitate, waver in opinion" (related to dubius "uncertain;" see dubious), originally "to have to choose between two things."

The sense of "fear" developed in Old French and was passed on to English. Meaning "to be uncertain" is attested in English from c. 1300. The -b- was restored 14c. by scribes in imitation of Latin. Replaced Old English tweogan (noun twynung), from tweon "two," on notion of "of two minds" or the choice of two implied in Latin dubitare (compare German Zweifel "doubt," from zwei "two").
doubt (n.)youdaoicibaDictYouDict
early 13c., from Old French dote (11c.) "fear, dread; doubt," from doter (see doubt (v.)).