The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]
   I . Un- is prefixed to adjectives, or to words used
     adjectively. Specifically:
     [1913 Webster]
   (a) To adjectives, to denote the absence of the quality
       designated by the adjective; as,
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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:
   (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.
       [1913 Webster]

   Note: Compounds of this prefix are given in full in their
         proper order in the Vocabulary.
         [1913 Webster]

WordNet (r) 3.0 (2006):

UN
    n 1: an organization of independent states formed in 1945 to
         promote international peace and security [syn: United
         Nations, UN]