Where "Cactus" Comes From
The Macquarie Dictionary traces Australian slang "cactus" — meaning broken, dead, or ruined — back to the prickly pear, an invasive cactus species that once covered enormous swathes of Australian farmland, so pervasive it became a byword for something unwanted and useless.
The prickly pear infestation was eventually brought under control in the 1920s by introducing the Cactoblastis moth as a biological pest control, one of the most successful examples of its kind anywhere in the world — giving a rough marker for when the plant, and the slang sense built on its reputation, would have been common knowledge across Australia. "The engine's cactus" and "he's in the cactus" both lean on that same sense of something thoroughly done for. The earliest print citation for the slang sense is 1967, from J. Wynnum's book "I'm a Jack, All Right."
Source:
"The car's cactus - won't start."
How Aussies Actually Use It
"Cactus" is a simple, blunt way to say something no longer works — a car, an appliance, a plan, even a relationship can all be described as cactus.
It's occasionally stretched to describe a person who's exhausted or worn out, though "broken or not working" remains by far its most common use.
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