Green’s Dictionary of Slang

cundum n.

[SE cundum or condom, a contraceptive sheath]

a false scabbard used to hide a sword.

[UK]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.
[UK]Grose Classical Dict. of the Vulgar Tongue (3rd edn).
[UK]Lex. Balatronicum.