What is a plausible etymology of "dosh", a British slang word for money?

Tony Thorne, The Dictionary of Contemporary Slang (1990) repeats one of the origin theories that Partridge notes (as quoted in user3286's answer), but also suggests an alternative involving "the African colonial term dash":

dosh n British[:] money. This is a working-class term from the early 1950s which was falling out of use in the 1960s, but which, like many similar words (bunce, loot, lolly, etc.), was revived in the money conscious late 1980s. It is a favourite with alternative comedians and 'professional cockneys'. The original would seem logically to be the old African colonial term dash, denoting a tip or bribe, but other authorities claim that it is influenced by doss [defined in its own entry as meaning "a place to sleep," "a period of sleep," or "a very easy task"], in the sense of the price of a bed (for the night).

John Ayto & John Simpson, The Oxford Dictionary of Modern Slang (1992), meanwhile, stick to the OED line of "origin unknown":

dosh noun Money 1953–. ** 'America! The money's in America!' ... ''Tis true. The Yankees have the dosh all right' (1970) {Origin unknown}