Green’s Dictionary of Slang

1. anything or anyone that is completely destroyed, beyond all hope of repair.

1918		J.M. Grider War Birds (1926) 96: Pansy ran into a chimney with a Camel and scored one complete write-off.
1919		W.H. Downing Digger Dialects 54: write-off [...] (2) Anything completely spoiled or broken; (3) a man who is killed.
1929		Hall & Niles One Man’s War 6: His plane was a total ‘write-off.’.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 105: My mum’s veins are approximately in the vicinity of a write off.
1968		D. O’Grady A Bottle of Sandwiches 106: It [a storm] snapped off one of the guy-wires, removed the tank, bashed and belted it [...] and left it down in the gully behind the house. A write-off.
1970		A. Sillitoe Start in Life (1979) 61: Far from being a write-off, it now seemed that [...] only one of the front lamps was smashed.
1985		T. Wolff ‘The Poor Are Always With Us’ in Back in the World 67: ‘[T]hey should lock him up and throw away the key [...] As far as I’m concerned he’s a complete write-off’.
1993		G. Disher Deathdeal [ebook] He had a shitty couple of days coming up—no reason why it had to be a total write-off.
1999		Indep. Rev. 21 July 8: The pushchair’s a write-off.
2000		Observer Rev. 26 Mar. 3: Apart from art and geology lessons, school was a write-off.
2006		B. Matthews Intractable [ebook] [T]he car was a write-off.

2. a farewell, a termination.

1973

G.V. Higgins Digger’s Game (1981) 71: This is the write-off and all.