The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         You can say and unsay things at pleasure. --Goldsmith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unsay
    v 1: take back what one has said; "He swallowed his words" [syn:
         swallow, take back, unsay, withdraw]