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.
可以想成一樣東西被用到最後一點都不剩。這個字背後指向的是 knacker's yard——以前把已經做不動的老牲口送去、不再使用的地方。抓住那個畫面，兩個用法就都跟著出來了：一個人忙完一整天，身上什麼都不剩；一台腳踏車或洗衣機，也走到了它的盡頭。
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, "knackered"
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