Something that has been used right to the end of what it had. The old word behind it points at the knacker's yard, the place worn-out working animals were sent when they could not work any more. Hold that picture and both uses fall out of it: a person with nothing left in the tank at the end of a long day, and a bike or a washing machine that has simply finished.
From the word’s history
From the verb knacker; the adjective is also traced to the phrases ready for the knacker's yard and fit to be knackered, said of worn-out livestock.
來自動詞 knacker；來源也把它接到 ready for the knacker's yard、fit to be knackered 這些說法，原本講的是已經做不動的牲口。
Wiktionary entries for knackered (kaikki:3f6f69a190036ae4:529380542 and :529390325), etymology sections supplied in the evidence packet
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