‘Knackered’ has come to mean exhausted or very tired… “I feel knackered” or “that car’s knackered”; there’s also a rhyming slang version – ‘cream crackered’.
In the early part of the twentieth century horse drawn carts were commonplace on British streets, and when the animal in question had outlived its useful working life (sometimes while pulling the cart – hence the folly of ‘flogging a dead horse’) then the knackerman would be called in.
The knackerman would collect and dispose of working animals at his workplace, ‘the knackers yard,’ processing the carcass. In this respect, they were the original recyclers; the animal’s skin would be tanned for leather, the flesh usually used for pet food, the hooves for glue, and the fats reduced to tallow for fertiliser and so on.