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un-

  1. not, lack of, the opposite of: unconcern, unreason, unwonted
  2. back; the reverse or removal of: added to verbs to indicate a reversal of the action and to nouns to indicate a removal or release of the thing mentioned or from the condition, place, etc. indicated, and sometimes used merely as an intensive [unbind, unfold; unbonnet, unbosom; unloose]

See un- in American Heritage Dictionary 4

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