Knights of the post, or false witnesses, said to attend Westminster Hall, and other courts of justice, ready to swear any thing for hire.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Ægrotat * After-ClapNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes