The Collaborative International Dictionary of English v.0.48:

Unreason \Un*rea"son\, n. [Pref. un- not + reason.]
   Want of reason; unreasonableness; absurdity.
   [1913 Webster]

   Abbot of Unreason. See Abbot of Misrule, under Abbot.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unreason \Un*rea"son\, v. t. [1st pref. un- + reason.]
   To undo, disprove, or refute by reasoning. [Obs.]
   [1913 Webster]

         To unreason the equity of God's proceedings. --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unreason
    n 1: the state of being irrational; lacking powers of
         understanding [syn: irrationality, unreason]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "unreason":
   callowness, childishness, immaturity, imprudence, inadvisability,
   inattention, inconsideration, indiscreetness, indiscretion,
   inexpedience, injudiciousness, irrationality, pompousness,
   puerility, reasonlessness, recklessness, senselessness, stuffiness,
   thoughtlessness, unintelligence, unreasonableness, unsensibleness,
   unsoundness, unthoughtfulness, unwisdom, unwiseness, witlessness