Green’s Dictionary of Slang

(Aus./N.Z.) thick-skinned, impervious to pain, lucky.

1937		Partridge DSUE 888/1: Tinny, adj. [...] Occ. tin-arsed.
1941		Baker Popular Dict. Aus. Sl. 76: tin-ars–ed: Unusually lucky.
c.1943		S .J. Baker Aus. Vulgarisms [t/s] 14: tinarsed: Lucky, especially when the luck is of a fantastic type. This term is the original of the colloquial tinny.
1957		‘Nino Culotta’ They’re a Weird Mob (1958) 72: Bob came out of ut all right, didn’ yer Bob? Tin arse Bob they call ’im.
1971		R.F. Brissenden Winter Matins 25: This tin-arsed character / Hasn’t been there six months before he starts / To fidget, gets to grizzling in his beer / About the wife and kids.
1986		R. Beckett Dinkum Aussie Dict. 52: Tin arsed: Lucky.
2002		Sydney Morning Herald 30 Nov. 🌐 Carl Hiaasen, The Miami Herald’s normally exhilarating novelist-columnist, recently harpooned tin-arsed Governor Jeb Bush for citing Florida’s population growth as a Good Thing.
2002		Tearaway.co.nz 30 Dec. 🌐 You know those tin-arse people who get everything they want? You think: ‘They’re so lucky, why do they get it?’ You know why? Because they’re always thinking about it and they’re grateful for it.