[1] : 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-. root193. 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
see also:
a- in- 

[2] : 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
see also:
answer anti- un- 

[3] : WordNet (r) 2.0

un
     n : an organization of independent states formed in 1945 to
         promote international peace and security syn: united
         nations
see also:
united nations 

[4] : THE DEVIL'S DICTIONARY ((C)1911 Released April 15 1993)

un-:american:, adj.  wicked, intolerable, heathenish.

Un- >> U Words
Un-, definition of term: Un-
un-_pag1.html