[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. What is meant by 'negative connotation'? Where is it?

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.