來自短語on less (than), 在少于的情況下，后由于該短語的否定含義，介詞on被改寫成了un.詞義演變比較despite, in spite of .
英文詞源
unless (conj.)
mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less. The first syllable originally on, but the negative connotation and the lack of stress changed it to un-. "Except could once be used as a synonym for unless, but the words have now drawn entirely apart" [Century Dictionary].
雙語例句
1. The targets would not be achieved unless other departments showed equal commitment.
除非其他部門同樣努力，否則這些目標無法達成。
來自柯林斯例句
2. Training is worthless unless there is proof that it works.
除非能證明訓練有效，否則就是白搭。
來自柯林斯例句
3. The South African mailboat arrived on Friday mornings unless bad weather intervened.
除非受惡劣天氣干擾，南非郵船一般在星期五上午抵達。
來自柯林斯例句
4. All bitches should be spayed unless being used for breeding.
除非是用作種犬，所有的母犬都應摘除卵巢。
來自柯林斯例句
5. I tend to forget things unless I mark them down.