A knacker is a person in the trade of rendering animals that are unfit for human consumption, such as work horses that have died in harness or are too tired to work any more. This leads to the slang expression "knackered" meaning very tired, or ready for the knacker’s yard, where old horses would be slaughtered and made into dog food and glue. The word is derived from the old Irish word for a horse (an each) pronounced a nack.