The Collaborative International Dictionary of English v.0.48:

Jack Ketch \Jack" Ketch"\ [Perh. fr. Jack, the proper name +
   Prov. E. ketch a hangman, fr. ketch, for catch to seize; but
   see the citations below.]
   A public executioner, or hangman. [Eng.]
   [1913 Webster]

         The manor of Tyburn was formerly held by Richard
         Jaquett, where felons for a long time were executed;
         from whence we have Jack Ketch.          --Lloyd's MS.,
                                                  British
                                                  Museum.
   [1913 Webster]

         [Monmouth] then accosted John Ketch, the executioner, a
         wretch who had butchered many brave and noble victims,
         and whose name has, during a century and a half, been
         vulgarly given to all who have succeeded him in his
         odious office.                           --Macaulay.
   [1913 Webster]