[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 optional info)

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

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

The less confuses me: in the conditional sentence 'A unless B', what's less about B?

Also, would someone please explain the 'negative connotation'?

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