ketch : Idioms & Phrases

(Naut.), a small ketch or vessel, very strongly built, on which mortars are mounted to be used in naval bombardments; called also mortar vessel.

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.