In Australian English, the slang expression grey death designates the unpalatable and unnutritious evening stew that used to be served to prison inmates.

The earliest occurrences of the expression grey death that I have found are as follows, in chronological order:

1-: From an interview of Walter Richard McGeechan, Commissioner of Corrective Services (New South Wales)—interview by Toni McRae, published in The Sun-Herald (Sydney, New South Wales, Australia) of Sunday 17^th February 1974 [page 41, column 1]:

McGeechan got into prisons in 1964—when there were about 17 institutions in NSW and some 665 prison officers to patrol them.
There were also, allegedly, things like the “Black Peter.” That meant putting a prisoner up for punishment, in a single cell for two or three days, with no clothes, two blankets at night and a diet of bread and water.
“Grey Death” was around in those days, too—a lump of fat in a bowl of luke-warm water—part of the ration.

2-: From ‘Who would listen to a crim?’, published in The Canberra Times (Canberra, Australian Capital Territory, Australia) of Friday 22^nd February 1974 [page 2, column 7]:
—In this text, a Canberra resident with a criminal record and 19 years’ experience of jail life discusses prison conditions:

“Prisons should be more open places with more open-air work and a reasonable feed for a reasonable day’s work. It would not cost a lot more to give prisoners a reasonable feed instead of the sort of grey death meals they get now.
“A prisoner working on building a new cell block should be entitled to a good feed. He will work better and you will cut down on a lot of the hostilities. Even the screws would be better for it.”

3-: From In jail the food of life may be a pound of flesh, published in The Eastern Herald (Sydney, New South Wales, Australia) of Thursday 15^th May 1986 [page 12, column 2]:
—In this text, an inmate of prisons since the late 1960s “reviews the a la carte menu at Long Bay Jail”:

It’s not just the security that’s overdone but usually the food as well. Whoever called this place “the Long Bay Hilton” must have made his observations from a distance because he couldn’t possibly have been here at a meal time.
The quality of the food ranges from poor to criminal, depending on the ability and temperament of the cooks. And the quantity follows the ups and downs of departmental fundings.
In the old days—way back in the 1960s—the prison fare consisted of porridge for breakfast, porridge for lunch, and a stew—affectionately called “grey death”—for dinner. That was seven days a week, 365 days a year.

4-: From Heroin: the jailhouse pacifier, about “the hard drugs pouring into the jails”, by Ben Hills, published in The Sydney Morning Herald (Sydney, New South Wales, Australia) of Saturday 8^th November 1986 [page 41, column 1]:

No-one really disputes the fact that heroin today is as much a part of prison life as “grey death” stew was to a generation of older lags.

By extension, the Australian-English expression grey death has come to be used of any unpalatable item of food. The following is from Fare thee well from Wimmera, by Des Ryan, published in the Hills & Valley Messenger (Port Adelaide, South Australia, Australia) of Wednesday 4^th June 2003 [page 2, column 1]:

I am no sentimentalist when it comes to home cooking.
My mother’s stew was a gristle pot; her tuna mournay was tuna mourning; and her minced meat pie was grey death. Strangely, my kids loved “nana’s pie” so there is no accounting for taste with some people.