I didn't say it wasn't true, Danny, only that I considered it dubious or that it was suspect. TOED scholars are perfectly happy to record the etymologies of 'plonk' and 'char' and a host of other words originating as military slang...the former being of Australian origin (possibly Australian military, as it first appears in 1919 just post World War I) and the latter being Hindi for 'tea', presumably brought back here by soldiers of the Raj.
Why, then, would they have failed to find your claimed source for 'dosh'? Do you actually have any documentary evidence - a sailor's letter home to his wife explaining his lack of money, say, pre-dating 1953 - for the ishky-dosh source? If so, I'd be delighted to learn where I might be able to see it and so, I am sure, would the Chief Editor at TOED!
I'm quite serious. I don't know whether you saw any of the recent TV series 'Balderdash & Piffle' but that made it perfectly plain that members of the public are more than welcome to submit evidence that might encourage the alteration of the dictionary if it is sufficiently persuasive. I'm sorry, but I'm not at the moment persuaded, since - as things stand - neither are the two major British dictionaries. Over to you. Cheers