A return response to an Aussie mate (male) usually over a few beers who says something so outrageous that “fair dinkum” just doesn’t cut it.
Mate at pub: Went fishing, fell in the water, got attached by a shark, managed to get out of the water fairly unscathed and scored a root on the walk back to the Ute.
Usually a term used to confirm the truth of a statement.
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.