The story
From a poem by Barcroft Boake, published in The Bulletin of 7 May 1892, about a fictional Jimmy Wood from Britain who is determined to end the practice of shouting (buying rounds of drinks for one′s group of mates). : One man one liquor! though I have to die A martyr to my faith, that′s Jimmy Wood, sir. Another mooted derivation is the Sydney slang term Johnny Warder (“a man who tries to cadge drinks”), after the similarly eponymous John Ward, a Sydney publican.
Etymology from Wiktionary, CC BY-SA 4.0.