Green’s Dictionary of Slang

tin-arse n.

also tin-back, tin-bum
[tin n. (1)+ arse n. (1)/SE back/bum n.1 (1)]

(Aus./N.Z.) an extremely lucky person.

[Aus]W.T. Goodge ‘Great Aus. Slanguage’ in Baker Aus. Lang. (1945) 117: And a tin-back is a party / Who’s remarkable for luck.
[Aus]D. Niland Shiralee 142: Tin-bum, they call me. I get on to opal [...] wherever I sink a shaft.
[NZ]G. Slatter Pagan Game (1969) 158: He’s been a bit of a tin-bum [...] Not like me. I’m always shit out of luck.
[Aus]Les Ryan Shearers 79: Good on yer, Joe. You alays were a tin-arse [GAW4].
[NZ]H. Beaton Outside In I i: Tin arse!
[NZ]McGill Dict. of Kiwi Sl. 113/1: tin arse lucky person; also tinbum.
[Aus]Age (Melbourne) 10 June [Internet] Trainer Colin Alderson said after De Gaulle Lane’s win: ‘Andrew is just a tin arse when it comes to picking these horses.’.