Green’s Dictionary of Slang

a professional witness who, with pay, will swear to anything.

1618		D. Digges Declar. Ambassador to Emperor of Russia [broadside]The foule exactions, extorsions, and base vsages towards prisoners, by [...] worthy instruments, seruants, and Affidavit men.
1681		E. Hickeringill The Horrid Sin of Man-Catching 7: It is a difficult thing to prove a Rogue perjur’d; and an Affidavit-man.
c.1698		B.E. Dict. Canting Crew n.p.: Affidavit men, Knights of the Post, Mercenary Sweaters for Hire, Inhabitants (formerly) of White Friers, now dispersed.
1699		N. Ward London Spy V 119: They will out-Wheedle a Gipsie, out-Swear a Common Gamester, out-lie an Affidavit Man, and out-Cozen a Tally-Man.
1719		A. Smith Lives of Most Notorious Highway-men, etc. (1926) 201: Affidavit-men, knights of the post, mercenary swearers for hire, inhabitants formerly of Whitefriars, now dispersed.
1725		New Canting Dict.
1737		Bailey Universal Etym. Eng. Dict. n.p.: affidavit men, Knights of the Post; mercenary and abandoned Wretches, who used to frequent the Temple and other Inns of Court, in order to be in Readiness to swear anything that was proposed to them.
1785		Grose Classical Dict. of the Vulgar Tongue.
1797		Sporting Mag. Sept. X 312/1: Common Bail, Affidavit Men, Ruffians, Bravoes, Assassins, multus cum aliis.
1811		Lex. Balatronicum.
1839		Manchester Courier 29 June 2/3: That’s the affydavy man [...] he’s the clerk of the swag, a knowing cove who looks out for the flats.
1862		E. de la Bédollière Londres et les Anglais 312/1: affidavit men, mercenaires qui sont prets a jurer n’importe quoi, devant n’importe quel tribunal; faux témoins.