Source : Webster's Revised Unabridged Dictionary (1913)

Unsay \Un*say\, v. t. [1st pref. un- + say.]
   To recant or recall, as what has been said; to refract; to
   take back again; to make as if not said.

         You can say and unsay things at pleasure. --Goldsmith.

Source : WordNet®

unsay
     v : take back what one has said; "He swallowed his words" [syn:
         {swallow}, {take back}, {withdraw}]
     [also: {unsaid}]