Origin and history of dubious

c. 1200, douten, duten, "to dread, fear, be afraid" (a sense now obsolete), from Old French doter "doubt, be doubtful; be afraid," from Latin dubitare "to doubt, question, hesitate, waver in opinion" (related to dubius "uncertain"), from duo "two" (from PIE root *dwo- "two"), with a sense of "of two minds, undecided between two things." Compare dubious. Etymologically, "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, hesitate or waver in opinion" is attested in English from c. 1300. The transitive senses of "be uncertain as to the truth or fact of" and "distrust, be uncertain with regard to" are from c. 1300.

The -b- was restored 14c.-16c. in French and English by scribes in imitation of Latin. French dropped it again in 17c., but English has retained it.

It replaced Old English tweogan (noun twynung), from tweon "two," on notion of "of two minds" or the choice between two implied in Latin dubitare. Compare German Zweifel "doubt," from zwei "two."

"doubtfulness, dubiousness," 1650s, from Late Latin dubietas "doubt, uncertainty," from Latin dubius "vacillating, fluctuating," figuratively "wavering in opinion, doubting" (see dubious). Earlier in the same sense were dubiosity (1640s), dubiousness (1650s); also see dubitation.

Ignorance is the mother of two filthy daughters; the first daughter of Ignorance is called dubiety, or doubtfulnesse, which is a continual wavering in opinion; a knowing man hath a fixt spirit, and settled judgement, but an ignorant man is a double-minded man, though he be never so resolute and wilful in his opinions. [W. Geering, "The Mischiefes and Danger of the Sin of Ignorance," London, 1659]