Green’s Dictionary of Slang

an absorbent plaster applied to buttocks and thighs that have been chafed by too much riding.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Cuirass a Dyachilon Palister, applied to the parts galled by riding. Quasi Cure A—se.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Cure A-se. A dyachilon plaister, applied to the parts galled by riding.
1811		Lex. Balatronicum.