Picture one thing lifted off the mould, and then the mould put away. There is that one piece, and there will be no second one. Every use of one-off grows out of that picture: the event held once, the offer nobody will repeat, the car built as a single example, and the person nobody else is quite like.
From the word’s history
Wiktionary derives it from one + off, and notes that it probably comes from foundry work, where moulds are expensive and normally reused. A one-off is the single piece taken off a mould that will not be used again. The same source says printmakers and sculptors have long used it for a unique print or casting, numbered 1/1.
維基詞典說 one-off 來自 one 加 off，可能出自鑄造工廠：模具很貴，通常會重複使用，而 one-off 指的是只從模子上取下那一個，不再做第二件。同一份資料也提到，版畫和雕塑界長期用 one-off 指「一版一件」，寫成 1/1。這條線索正好接上它今天的用法。
Wiktionary 'one-off'（封包 source_ref kaikki:3f6f69a190036ae4:917272632）
登入就可以看每一支怎麼連回核心畫面。