[Etymonline:] unless (conj.) [:]
mid-15c., earlier onlesse, from on lesse (than) "on a less 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].
[Source:] The only (very slight) semantic shift I can see is that originally the condition was effectively a minimum (the LEAST thing that's required), but nowadays it's normally used without the speaker being consciously aware of that original nuance
(now it's usually the ONLY thing that will do).
Despite the above, I still don't understand what is less in 'unless'. What does condition mean above? Does it refer to the clause before 'unless', 'unless' itself, or the clause after 'unless'?
Footnote: I changed the formatting of the original quote for clarity, but not the text.
I don't quote the brusquer OED, which doesn't discuss such a condition.