来自短语on less (than), 在少于的情况下，后由于该短语的否定含义，介词on被改写成了un.词义演变比较despite, in spite of .
英文词源
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].
实用例句
1. The targets would not be achieved unless other departments showed equal commitment.
除非其他部门同样努力，否则这些目标无法达成。
来自柯林斯例句
2. Training is worthless unless there is proof that it works.
除非能证明训练有效，否则就是白搭。
来自柯林斯例句
3. The South African mailboat arrived on Friday mornings unless bad weather intervened.
除非受恶劣天气干扰，南非邮船一般在星期五上午抵达。
来自柯林斯例句
4. All bitches should be spayed unless being used for breeding.
除非是用作种犬，所有的母犬都应摘除卵巢。
来自柯林斯例句
5. I tend to forget things unless I mark them down.
