Green’s Dictionary of Slang

1. rum.

1984		Partridge DSUE (8th edn) 647: C.20.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.

2. the buttocks [= bum n.¹ (1)].

1979		R. Barker Fletcher’s Book of Rhy. Sl. 43: What she needed was a good smack on her Kingdom come.
2003		B. Dark Dirty Cockney Rhy. Sl.