Green’s Dictionary of Slang

(Aus.) dead, dying.

1823		Sydney Gaz. 30 Oct. 4/2: Bill has plenty of stock, and will have a good farm, when father’s off.
[1828		G. Smeeton Doings in London 157: The present Jack Ketch is Ould Tom Cheshire [...] It was intimated to him, for ‘Tom has always an eye to business,’ that one of the culprits had a good watch [...] ‘Then it belongs to me, and as soon as he’s off, I’ll bone it’].
[1838		‘Nights At Sea’ in Bentley’s Misc. Apr. 593: Andy! They’ve knocked daylight through me! I’m off, ould boy!].
1991		M.B. ‘Chopper’ Read Chopper From The Inside 137: You fat slug, you’re off as well. I’ll fix you all! When Chopper gets back you’re a dead man.