Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • Not: unhappy.
  • Opposite of; contrary to: unrest.
  • To reverse or undo the result of a specified action: unbind.
  • To deprive of or remove a specified thing: unfrock.
  • To release, free, or remove from: unyoke.
  • Used as an intensive: unloose.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • not
  • absent, lacking, not
  • reverse, opposite
  • release, free, remove, extract.
  • Used to form temporary names of elements (such as ununbium) whose existence has been predicted, and have not yet been given a systematic name.

from the GNU version of the Collaborative International Dictionary of English

  • An inseparable verbal prefix or particle. It is prefixed: (a) To verbs to express the contrary, and not the simple negative, of the action of the verb to which it is prefixed; as in unbend, uncoil, undo, unfold. (b) 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.
  • 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.
  • Un- is prefixed to adjectives, or to words used adjectively.
  • To adjectives, to denote the absence of the quality designated by the adjective.
  • 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.

Etymologies

Middle English, from Old English.
Middle English, from Old English on-, alteration (influenced by un-, not) of ond-, and-, an-, against, opposing.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From Middle English un-, from Old English un- ("un-"), from Proto-Germanic *un- (“un-”), from Proto-Indo-European *n̥- (“un-, not”). Cognate with Scots un-, on- ("un-"), North Frisian ün- ("un-"), Saterland Frisian uun- ("un-"), West Frisian ûn-, on-, Dutch on- ("un-"), Low German un-, on- ("un-"), German un- ("un-"), Danish u- ("un-"), Swedish o- ("un-"), Norwegian u- ("un-"), Icelandic ó- ("un-"). Related also to Latin in-, Ancient Greek ἀ- (modern Greek α-) and Sanskrit अ- (a-). (Wiktionary)
From Middle English on-, from Old English ond-, and-. More at and-. (Wiktionary)
From Latin ūnus. (Wiktionary)

Examples

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