Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Abbot of Unreason}. See {Abbot of Misrule}, under {Abbot}.

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

         To unreason the equity of God's proceedings. --South.

Source : WordNet®

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