Well, shut my "bone box," they would have needed "bum fodder" (soft paper for
the necessary house) and that's likely because of the "piss prophet?" ("A
physician who judges of the diseases of his patients solely by the inspection
of their urine.")

"The Scoundrel's Dictionary: a copious and complete compendium of 18th
century Slang"
Compiled by Michelle Lovric, Adaptation copyright 1997, PAST TIMES, Oxford,
England

I could not find any reference here but a suggested source from the above
introduction may be "The Classical Dictionary of the Vulgar Tongue," 1785 by
Captain Francis Grose.