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]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "unsay":
   abjure, back down, back out, backwater, climb down, crawfish out,
   deny, disavow, disclaim, disown, eat crow, eat humble pie,
   forswear, palinode, recall, recant, renege, renounce, repudiate,
   retract, revoke, swallow, take back, withdraw