I can remember using the description "tin bum" in the 1960s (mostly in Victoria) and Sandra and Bob have mentioned the most likely traditional links to the term but there is another context that may (or may not) have some relevance.

In my youth I had a girlfriend whose family came from Newstead, in the goldfields of Central Victoria. On one of my visits there an old codger took a bit of a shine to me and wanted me to join him in a 'serious gold mining venture'. Well, I knew enough folk lore even then to avoid being sucked too far into such thgings but he did teach me how to use an ordinary long-handled shovel as a gold-panning dish (as well as a frying pan); this was a favourite way to spend smoko when out with the road-mending gang.

According to him (and I've verified it in other areas where alluvial gold occurs) the best way to check that your panning technique is 'on the money' is to find specks of tin in the bottom of the dish. Its specific gravity isn't as high as gold's but it's higher than anything else naturally in the alluvium so, if you find tin but no gold, you know you're technique is good. In areas that have never been National Parks, you could also expect to find lead shot.

Just panning on spec and turning up tin is a lucky start.

Cheers, Rowan