What does "Fair dinkum" mean?
It means genuine or true, and can emphasise that you really mean something. Said as a question, it can mean are you serious, often with surprise or disbelief.
The saying in a sentence
The saying
Fair dinkum
Example
Fair dinkum, that's the biggest fish I've seen.
When you use it
Use it in a casual conversation to stress that something is real, or to check whether a surprising story is true.
When you do not
Avoid it in formal writing, where genuine or really is clearer. Said sharply as a question, it can sound as though you doubt the other person.