Brewer's: Knights o' the Post
Persons who haunted the purlieus of the courts, ready to be hired for a bribe to swear anything; so called from their being always found waiting at the posts which the sheriffs set up outside their doors for posting proclamations on.
“There are knights of the post and booby cheats enough to swear the truth of the broadest contradictions.” —South.
“ `A knight of the post,' quoth he, `for so I am termed; a fellow that will sweare you anything for twelve pence.' ” —Nash: Pierce Penilesse (1592.)