Secret training for Saturday Matt
According to Melvyn Bragg's The Adventure of English: The Biography of a Language, "dinkum" comes from the English Midlands and meant work. "Fair dinkum" referred to a fair day's work and subsequently fair play.
M. Griffiths, Waterloo
The word "dinkum" was reputedly coined on the Australian goldfields. It comes from one of the Chinese dialects widely spoken at the diggings: "din" and "kum" loosely translating as "true gold".
Catherine Le Breton, Leura
Fair Dinkum was a response of the early Chinese goldminers to the question: "Are you finding a fair amount of gold?" because "din-gum" means "good gold". So over time the expression has become a positive response to a good news story