The Collaborative International Dictionary
Un- \Un-\ [OE. un-, on-, the unaccented form of the accented prefix and- (cf. Answer); akin to D. ont-, G. ent-, OHG. int-, Goth. and-. See Anti-.] An inseparable verbal prefix or particle. It is prefixed:
To verbs to express the contrary, and not the simple negative, of the action of the verb to which it is prefixed; as in uncoil, undo, unfold.
To nouns to form verbs expressing privation of the thing, quality, or state expressed by the noun, or separation from it; as in unchild, unsex. Sometimes particles and participial adjectives formed with this prefix coincide in form with compounds of the negative prefix un- (see 2d Un-); as in undone (from undo), meaning unfastened, ruined; and undone (from 2d un- and done) meaning not done, not finished. Un- is sometimes used with an intensive force merely; as in unloose.
Note: Compounds of this prefix are given in full in their proper order in the Vocabulary.
Un- \Un-\ [OE. & AS. un-; akin to OFries. un-, D. on-, OS., OHG., & G. un-, Icel. [=o]-, [=u]-, Sw. o-, Dan. u-, W. an-, L. in-, Gr. ?, ?, Skr. an-, a-. [root]193. Cf. A- not In- not, No, adv.] An inseparable prefix, or particle, signifying not; in-; non-. In- is prefixed mostly to words of Latin origin, or else to words formed by Latin suffixes; un- is of much wider application, and is attached at will to almost any adjective, or participle used adjectively, or adverb, from which it may be desired to form a corresponding negative adjective or adverb, and is also, but less freely, prefixed to nouns. Un- sometimes has merely an intensive force; as in unmerciless, unremorseless. I . Un- is prefixed to adjectives, or to words used adjectively. Specifically: (a) To adjectives, to denote the absence of the quality designated by the adjective; (b) To past particles, or to adjectives formed after the analogy of past particles, to indicate the absence of the condition or state expressed by them; (c) To present particles which come from intransitive verbs, or are themselves employed as adjectives, to mark the absence of the activity, disposition, or condition implied by the participle; Note: The above classes of words are unlimited in extent, and such compounds may be formed by any writer or speaker at will from almost all the adjectives or participles in the language, excepting those which have a recognized and usual negative correspondent with the prefix -in. No attempt will be made, therefore, to define them all in this Dictionary; many will be omitted from its Vocabulary which are negations of the simple word, and are readily explained by prefixing a not to the latter. Derivatives of these words in -ly and -ness will also, for the most part, be omitted for the same or similar reasons. There will be inserted as separate articles with definitions, the following:
Those which have acquired an opposed or contrary, instead of a merely negative, meaning; as, unfriendly, ungraceful, unpalatable, unquiet, and the like; or else an intensive sense more than a prefixed not would express; as, unending, unparalleled, undisciplined, undoubted, unsafe, and the like.
Those which have the value of independent words, inasmuch as the simple words are either not used at all, or are rarely, or at least much less frequently, used; as, unavoidable, unconscionable, undeniable, unspeakable, unprecedented, unruly, and the like; or inasmuch as they are used in a different sense from the usual meaning of the primitive, or especially in one of the significations of the latter; as, unaccountable, unalloyed, unbelieving, unpretending, unreserved, and the like; or inasmuch as they are so frequently and familiarly used that they are hardly felt to be of negative origin; as, uncertain, uneven, and the like.
Those which are anomalous, provincial, or, for some other reason, not desirable to be used, and are so indicated; as, unpure for impure, unsatisfaction for dissatisfaction, unexpressible for inexpressible, and the like. II . Un- is prefixed to nouns to express the absence of, or the contrary of, that which the noun signifies; as, unbelief, unfaith, unhealth, unrest, untruth, and the like.
Note: Compounds of this last class are given in full in their proper order in the Vocabulary.
Douglas Harper's Etymology Dictionary
prefix of reversal, deprivation, or removal (as in unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (cognates: 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, against" (see ante).\n
\nMore or less confused with un- (1) through similarity in the notions of "negation" and "reversal;" an adjective such as unlocked might represent "not locked" (un- (1)) or the past tense of unlock (un- (2)).
prefix of negation, Old English un-, from Proto-Germanic *un- (cognates: 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-), comb. form of PIE root *ne "not" (cognates: Avestan na, Old Church Slavonic and Lithuanian ne "not," Latin ne "that not," Greek ne- "not," Old Irish ni, Cornish ny "not"). Often euphemistic (such as untruth for "lie"). \n
\nThe 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.\n
\nIt 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.
Etymology 1 pre. 1 (context added to adjectives or past participles English) not 2 (context added to nouns English) absent, lacking, not Etymology 2
pre. 1 (context added to verbs and nouns to form verbs English) reverse, opposite 2 release, free, remove, extract. Etymology 3
pre. Used to form temporary names of elements (such as ununbium) whose existence has been predicted, and have not yet been given a systematic name.