a prefix meaning “not,” freely used as an English formative, giving negative or opposite force in adjectives and their derivative adverbs and nouns (unfair; unfairly; unfairness; unfelt; unseen; unfitting; unformed; unheard-of; un-get-at-able ), and less freely used in certain other nouns (unrest; unemployment ).
Compare un-2.

Middle English un-, on-, Old English; cognate with Dutch on-, Gothic, German un-, Old Norse ū-, ō-; akin to Latin in-, Greek an-, a-. See a-6, an-1, in-3

See in-3.
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a prefix freely used in English to form verbs expressing a reversal of some action or state, or removal, deprivation, release, etc. (unbend; uncork; unfasten , etc.), or to intensify the force of a verb already having such a meaning (unloose ).
Compare un-1.

Middle English, Old English un-, on-; cognate with Gothic and-, Dutch ont-, German ent-; akin to Latin ante, Greek antí; cf. ante-, anti-

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
(freely used with adjectives, participles, and their derivative adverbs and nouns: less frequently used with certain other nouns) not; contrary to; opposite of: uncertain; uncomplaining; unemotionally; untidiness; unbelief; unrest; untruth
[from Old English on-, un-; related to Gothic on-, German un-, Latin in-]

prefix forming verbs and verbal derivatives
1.  denoting reversal of an action or state: uncover; untangle
2.  denoting removal from, release, or deprivation: unharness; unman; unthrone
3.  (intensifier): unloose
[from Old English un-, on-; related to Gothic and-, German ent-, Latin ante]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

prefix of negation, O.E. un-, from P.Gmc. *un- (cf. O.Fris., O.H.G., Ger. un-, Goth. un-, Du. on-), from PIE *n- (cf. Skt. a-, an- "not," Gk. a-, an-, O.Ir. an-, L. in-), a variant of PIE base *ne- "not" (cf. Avestan na, O.C.S., Lith. ne "not," L. ne "that not," Gk. ne- "not," O.Ir. ni, Corn. ny "not").
Freely and widely used since O.E. times in compounds with native and imported words, it disputes with Latin-derived cognate in- the right to form the negation of certain words (indigestable/undigestable, etc.). Often euphemistic (e.g. untruth for "lie"). The most prolific of Eng. prefixes, it even is used to make words from phrases (e.g. uncalled-for, 1610; undreamed-of, 1636; uncome-at-able, 1694; unputdownable, 1947, of a book). As a prefix in telegram-ese to replace not and save the cost of a word, it is first attested 1936.

prefix of reversal (e.g. unhand, undo, unbutton), O.E. on-, un-, from P.Gmc. *andi- (cf. O.S. ant-, O.N. and-, Du. ont-, O.H.G. ant-, Ger. ant-, Goth. and- "against"), from PIE *anti "facing opposite, near, in front of, before" (see ante).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

un- 1
Not: unmyelinated.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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