Green’s Dictionary of Slang

1. the vagina.

1675		C. Cotton Scoffer Scoff’d (1765) 231: So her Bumfiddle I had clapt, I’d be contented to be trapt.
1682		‘Joan Has Been Galloping’ in Farmer Merry Songs and Ballads (1897) II 84: Joan has been Galloping all the Town o’re, / ’Till her Bumfiddle [...] was wonderous sore.
1684–95		Jovial May Pole Dancers n.p.: Susan she clasped her William by the middle, And William he all to beclaw’d her Bumfiddle.
a.1701		N. Ward Revels of the Gods 8: For shame, my Fair Goddesses, bridle your Passions, / And make not In Heaven such filthy Orations / About your Bumfiddles.
1702		T. Brown Letters from the Dead to the Living in Works (1760) II 201: A pretty woman now-a-days may make a slave of her bumfiddle for thirty year together, and not get money enough to keep her out of an hospital [...] at the age of fifty.

2. the anus, esp. when it breaks wind.

1701		N. Ward ‘A Walk to Islington’ in Writings (1704) 72: But fir’d with the touch of this Sattin Bumfiddle, / The Dart of the God, prick’d my Heart like a Needle.
1709		N. Ward A Compleat and Humorous Account of all the Remarkable Clubs (1756) 14: The Inhabitants [...] were forced, when they went to bed, to wear Plugs in their Fundaments, to keep their laxative Bum-fiddles from dishonouring their Sheets.
1722		Benefit of Farting 7: That Musical Part had its Name of Bum-Fiddle.
1785		‘Mistress Stitch in Clover’ in Nightly Sports of Venus 30: Hob in his breeches went to bed, And Mistress Stitch was in the middle, Her face turn’d close to Bodkin’s head, To leather breeches her 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.