Green’s Dictionary of Slang

jumble n.

[pron. of John Bull; used by West African immigrants/students in UK and popularized in the books of Colin MacInnes (1914–76)]

a white person.

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[UK]C. MacInnes ‘A Short Guide for Jumbles’ in England, Half Eng. (1960) 19: What is a jumble? You are and I, if we are white. The word’s a corruption of ‘John Bull’, and is used by West Africans of Englishmen in a spirit of tolerant disdain.