Green’s Dictionary of Slang

the anus, esp. when it breaks wind.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Ars musica, a bum fiddle.
1793		Sporting Mag. Jan. I 224/1: On coming near to that [school] of music, on which is written ‘Ars Musica,’ a lady asked him what those words meant – ‘Bum-fiddle, to be sure, Madam,’ replied he.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1890–1904		Farmer & Henley Sl. and Its Analogues.