Chapter J to Jackknife of J by Webster's Dictionary (1913 Edition)

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.]

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.

[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.

Jackknife
(Jack"knife`) n. A large, strong clasp knife for the pocket; a pocket knife.