The Collaborative International Dictionary of English v.0.48:

Forthink \For*think"\, v. t.
   To repent; to regret; to be sorry for; to cause regret.
   [Obs.] "Let it forthink you." --Tyndale.
   [1913 Webster]

         That me forthinketh, quod this January.  --Chaucer.
   [1913 Webster]