[Etymonline:] 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-. ...

Source: p 139, Introduction to Logic (2 ed, 2010) by Harry J. Gensler (More at Linguistics)

“Unless” is also equivalent to “if not”...

1. How did  on a less condition (than)  evolve into  onlesse , and finally,  if not?

2. Which condition is less? Does unless connote inferiority?

3. What is meant by, and where is the, 'negative connotation'?

Please help me dig deeper than the definitions above, which I already understand and so ask NOT about. I recognise the Etymological Fallacy. OED doesn't explain.