Entries linking to undoubted
prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").
The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.
It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.
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."