>[[Etymonline:]](http://www.etymonline.com/index.php?allowed_in_frame=0&search=unless&searchmode=none) mid-15c., earlier *onlesse*, from *on lesse* (than) "`on a less condition (than)`; see [less](http://www.etymonline.com/index.php?term=less&allowed_in_frame=0). The first syllable originally [on](http://www.etymonline.com/index.php?term=on&allowed_in_frame=0), 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 (cp [Linguistics](http://linguistics.stackexchange.com/q/12238/5306) or [Phil.](http://philosophy.stackexchange.com/q/23503/8572))

>“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.