More on Knight of the Post from Infoplease:

• Knight of the Post - Knight of the Post A man in the pillory, or that has been tied to a whipping-post, is jestingly so ...
• Knights o' the Post - Knights o' the Post Persons who haunted the purlieus of the courts, ready to be hired for a ...
• Dictionary of Phrase and Fable: K - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "K"