Green’s Dictionary of Slang

johnny bum n.

a donkey.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Johnny bum, a he or jack ass; so called by a lady that affected to be extremely polite and modest, who would not say jack, because it was vulgar, nor ass because it was indecent.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.