Green’s Dictionary of Slang

a false scabbard used to hide a sword.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Cundum [...] Also a false Scabbard over a Sword, & the Oil Skin Case for holding the Colours of a Regiment.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn).
1811		Lex. Balatronicum.