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un-1

1.
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 .
Origin
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
Synonym Study
See in-3.

un-2

1.
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 .
Origin
Middle English, Old English un-, on-; cognate with Gothic and-, Dutch ont-, German ent-; akin to Latin ante, Greek antí; cf. ante-, anti-
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for un-

un-1

prefix
1.
(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
Word Origin
from Old English on-, un-; related to Gothic on-, German un-, Latin in-

un-2

prefix
1.
denoting reversal of an action or state: uncover, untangle
2.
denoting removal from, release, or deprivation: unharness, unman, unthrone
3.
(intensifier): unloose
Word Origin
from Old English un-, on-; related to Gothic and-, German ent-, Latin ante
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for un-

prefix of negation, Old English un-, from Proto-Germanic *un- (cf. Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (cf. Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), a variant of PIE root *ne- "not" (cf. Avestan na, Old Church Slavonic and Lithuanian ne "not," Latin ne "that not," Greek ne- "not," Old Irish ni, Cornish ny "not").

Freely and widely used since Old English 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 English prefixes, it even is used to make words from phrases (e.g. 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 not restricted to un-; cf. put-up-able-with, 1812). 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), Old English on-, un-, from Proto-Germanic *andi- (cf. Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ant-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before" (see ante).

Online Etymology Dictionary, © 2010 Douglas Harper
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un- in Medicine

un- 1
pref.
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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2
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