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 any thing that was proposed to them.
Affidavit Men in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).