David, Dave B is on the money (pun intended) with the origin of Fair Dinkum.
The origins of other terms are not as clear, or are disputed. Dinkum or fair dinkum means "true", "the truth", "speaking the truth", "authentic" and related meanings, depending on context and inflection. It is often claimed that dinkum was derived from the Cantonese (or Hokkien) ding kam, meaning "top gold" or "deposit", during the Australian goldrushes of the 1850s. This, however, is chronologically improbable since dinkum is first recorded in the 1890s. Scholars give greater credence to the notion that it originated with a now-extinct dialect word from the East Midlands in England, where dinkum (or dincum) meant "hard work" or "fair work", which was also the original meaning in Australian English.[1] The derivation dinky-di means a 'true' or devoted Australian. The words dinkum or dinky-di and phrases like true blue are widely purported to be typical Australian sayings, however these sayings are more commonly used in jest or parody rather than as an authentic way of speaking.
I have heard the expression "no duff", but more common usage in OZ is
" she is up the Duff" an expression that strikes fear into Teenagers. Meaning she is pregnant.:stunned:
