1400-50; late ME prepositional phrase on less (also o less(e), earlier upon less) on a lesser footing or condition (than); first used as a preposition and conjunction in the early 16th century; see on, less
Examples from the web for unless
Water weekly for the first few months unless there is regular rain.
And don't use a food processor on any potato unless you want glue.
There should be a law against people buying old houses unless they plan to restore them.
Predators typically give skunks a wide berth unless little other food is available.
unless you've been living in a cave, you probably haven't run across this new species of poisonous, nearly blind pseudoscorpion.
Avoid bringing new hiking boots unless you have time to break them in beforehand.
The birds pale in captivity unless their diet is supplemented.
We will never share your e-mail address with other organizations unless you allow us to do so.
Don't open the camera back unless you are in an area that is well sheltered.
Personalized items cannot be returned unless damaged or defective.
British Dictionary definitions for unless
unless
/ʌnˈlɛs/
conjunction
1.
(subordinating) except under the circumstances that; except on the condition that: they'll sell it unless he hears otherwise
preposition
2.
(rare) except
Word Origin
C14: onlesse, from onon + lesseless; compare French à moins que, literally: at less than
Word Origin and History for unless
conj.
mid-15c., earlier onlesse, from on lesse (than) "on a less condition (than)," the first syllable originally on, but the negative connotation and the lack of stress changed it to un-.