What is 'less' about 'unless'?

Question

[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.

Answer 1

You would do well to read OED more carefully. OED 1's definition for the earliest meaning is

  †1. prep.phr.  On a less or lower condition, requirement, footing, etc., than what is specified.
  With preceding negative, express or implied. [my emphasis]

That is, the sense is "not on a less compelling condition than" what follows, NOT "On a less compelling condition than" what follows.

In modern terminology, on less (like modern unless) is a Negative Polarity Item: it only occurs in a negative context.

Here's the first quotation OED 1 gives, in modern spelling.

c 1400 [...] But that may not be upon less than we may fall toward heaven.

That is, "That cannot happen on any less condition than our being able to fall upwards."

1475 [...] His land, which many people .. fear to take .., unless than they might be made very sure of payment.

That is, people will not take the land on any less condition than that of being assured that they will be paid. Here fear supplies the necessary negative context.