G'day Helen,

The simple source is "tin" meaning money (specifically silver coins - so we aren't talking riches ... just reasonable money for working people. The Australian National Dictionary gives:

2. Special Comb. tin arse (~ back,~ bum), an unusually lucky person; also as tin-arsed. Their first citation is:

The Bulletin, 4 June 1898 with the "Red Page" defining a 'tin-back' as "a party who's remarkable for luck".

"Tinny" is simply a polite evasion of the pithier early forms.

We may have the Macquarie Dictionary as a 'national standard'... but if you want a real 'dictionary on historical principles', in the tradition of the OED, you must have >i>The Australian National Dictionary, from the successors to Oxford's (~) "Australian Language Centre" ... which was working on this long before I made some (very minor) submissions on Tasmanian variants in 1964!

Regards,

Bob