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
If one is single, not in a monogamous relationship, or otherwise not exclusively attached, they are said to be fairdinkum. (it's fair to dink 'um)
To pork someone who is not 'available' would be unfairdinkum.
Linda: "Tom is so hot, I wanna ride him like a Jersey steer. Too bad he's dating Cindy... she'd rip my uterus out with her bare hands if she found out I boned him."
Keri: "You're in luck! As of last night, Tom and Cindy are on a 'break' ... he's fairdinkum!"