The origins of this term are disputed but a plausible one is that it came from the Victorian Goldfields where Chinese workers used the term Ding kum so confirm a deal that is honest and true.
2)Statement of acknowledgement.
Similar term to "No Sh!t"
Person B - Fair Dinkum?
2)Person A - It's going to cost me lot of money to fix the car.
Person B - Fair Dinkum