Fair dinkum is one of Australia's favourite slang terms and is used in a variety of situations:
“Fair dinkum, you scared me!” Someone might say after you jumped out at them in the dark.
“Fair dinkum?” You might ask if you don’t know if someone is telling you the truth.
It originated in the Australian gold fields. The Chinese described their gold in their language as “Din” and “Kum”, meaning good gold or true gold. So it became a good news term to all the desperate miners. “Yeah, mate, it’s fair dinkum alright!”