Etymology Unknown. Earliest recorded use in the phrase fair dinkum in 1879, from Australia. Also recorded (slightly later) from Derbyshire and Lincolnshire dialect. Perhaps related to Gloucestershire dialect ding (“to work hard”).
Hon. Sir Ross McLarty: We were always dinkum. Mr. MAY: The ultimate result showed how dinkum the hon. member was.1956, parliamentary debates (Western Australian Parliament), page 2194:
The dinkum Aussie everyone talks about, almost always with a certain unreal sentimentality, is clearly a worker.1966, Craig McGregor, Profile of Australia, page 21:
2 more examples
Larry's jeep was behind mine, and as I went past an intersection, I saw a lot of their chaps around a dinkum super Mark VI tank — p'raps a Mark VII or VIII. I didn't have a chance for a proper dinkum look-see, what?2004, A. J. Liebling, Direction: Paris: Mollie and Other War Pieces, page 242:
He explained that he was due to have a game of hazards that night with a couple of Italian prospectors and that he was doctoring the dice so that they would do just what he wanted them to do. ‘Tim, is this game dinkum?’ asked Ted. Highly indignant that such a suspicion should arise, he replied angrily: ‘Of course it's dinkum. They'll have loaded dice too!’2006, Ron Fitch, Australian Railwayman: From Cadet Engineer to Railways Commissioner, page 65:
Etymology Unknown. Earliest recorded use in the phrase fair dinkum in 1879, from Australia. Also recorded (slightly later) from Derbyshire and Lincolnshire dialect. Perhaps related to Gloucestershire dialect ding (“to work hard”).