[uhn-les, uhn-]
except under the circumstances that: I'll be there at nine, unless the train is late.
except; but; save: Nothing will come of it, unless disaster.

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

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World English Dictionary
unless (ʌnˈlɛs)
1.  (subordinating) except under the circumstances that; except on the condition that: they'll sell it unless he hears otherwise
2.  rare except
[C14: onlesse, from onon + lesseless; compare French à moins que, literally: at less than]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1467, earlier onlesse (c.1440), 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-.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
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
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