Green’s Dictionary of Slang

the buttocks, the posterior.

1942		in G. Legman Limerick (1953) 56: His muse was the bum of a bird / And his Lesbian wife / Would finger his fife.
1960		J. Franklyn Dict. of Rhy. Sl.
1971		J. Jones Rhy. Cockney Sl.
1988		J. McDonald Dict. of Obscenity etc.
2002		M. Coles More Bible in Cockney 124: You sit there on your big fife-and-drum judging me according to the Law.