Unless

google

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late Middle English: from on or in (assimilated through lack of stress to un-1) + less.

wiktionary

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onlesse, from  on lesse, from "on a less condition" ("on" became  un-). 

etymonline

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unless (conj.)

mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less. The first syllable originally was on, but the quality of negation in the word 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].