Definition of tin arse in English:

tin arse

noun

Australian, NZ
informal
  • An exceptionally lucky person.

    ‘I'd be happy to be a tin arse for just one day at the races’
    • ‘Bob came out of it all right, didn' yer Bob? Tin arse Bob they call 'im.’
    • ‘Never praise a good shot – 'Tin-arse!' is the highest praise allowed.’
    • ‘His reputation as a tin arse lasted right to the end of his stay in the States.’
    • ‘Luke couldn't believe my tin arse since I had picked the winner 'again' when we were betting together.’
    • ‘Have been a total tin arse in the last year, Dad must be sending me luck!’

Origin

Late 19th century: from tin ‘money’, figuratively ‘luck’.