hey Lorraine, us Aussies use Knackered the same way you guys do but its original meaning is this (from Wikki):
A
knacker /ˈnæk.ə(ɹ)/ is a person in the trade of
rendering animals that have died on farms or are unfit for human consumption, such as
horses that can no longer work.
[1] This leads to the
slang expression "knackered" meaning very tired, or "ready for the knacker's yard", where old horses are slaughtered and the by-products are sent for rendering. A knacker's yard or knackery is different from a
slaughterhouse, where animals are slaughtered for human consumption. In most countries, knackery premises are regulated by law.
A horse carcass, rendered, had many uses. In the U.S., the meat could be used as food at a mink ranch, pig farm, fox farm, or greyhound race track, in pet food, or in zoos. Bones were ground up for bone meal fertilizer. Hides were made into leather or, along with joints and hooves, processed to make
glue for the furniture and book binding trades (hence the idea of old horses being sent to the glue factory).
there's more if you wanted to have a good look