Green’s Dictionary of Slang

coach n.1

In phrases

take a coach (v.)

(UK und.) to cut someone’s throat.

A Perfect Narrative of the Robbery [etc] 38: That these kind of persons are reported to have a Canting Language amongst themselves; as when they say take Coach, is meant cut a Throat; take Horse, take a Purse; Mill a Gruntling, steal a Pigg, &c.