Origin of “Fair Dinkum”: I’m afraid you’re both wrong, it is nothing to do with Latin or Chinese.
From http://www.abc.net.au/classic/breakfast/stories/s994019.htm and http://www.abc.net.au/wordmap/rel_stories/talkback.htm :
"Another folktale with great currency at the moment is about the origin of “fair dinkum”. It is supposed that “dinkum” is a Cantonese word meaning true gold and comes from the excited cry of Chinese on the goldfields when they struck it rich. The Europeans picked the word up in that linguistic cross-fertilisation that occurred when all manner of people were thrown together in the camaraderie of frontier life.
The origin of “dinkum” actually lies in British dialect, where it meant “a day’s allocation of work”. “Your dinkum” was what you were required to get done so, naturally. a fair dinkum was a matter of some importance. The phrase “fair dinkum” existed in the Lincolnshire dialect.
In ‘Robbery Under Arms’, Rolf Boldrewood writes, “It took us an hour’s hard dinkum to get near the peak”. From this came fair dinkum – originally meaning “a fair day’s work for a fair day’s pay”. And then the meaning broadened to the one we know today."