The Collaborative International Dictionary of English v.0.48:

Unkiss \Un*kiss"\, v. t. [1st un- + king.]
   To cancel or annul what was done or sealed by a kiss; to
   cancel by a kiss. [Obs.]
   [1913 Webster]

         Let me unkiss the oath 'twixt thee and me. --Shak.
   [1913 Webster]