I don't really see how the [etymological fallacy][1] (the mistaken belief that the "true" or "proper" meaning of a word is its oldest or original meaning) relates to current usage of unless.

OED's "etymology" for the word says...

less ... with the prepositions of, in, upon, and on ; the last of these by want of stress¹ has been assimilated in form to the prefix un-

Semantically, the specific preposition is fairly irrelevant. Casual/dialectal speakers commonly discard it completely (I have no opinion on whether the apostrophe should be present)...

["I won't go 'less you do, Josh," said Lester.][2]
["You don't have to go less you want to."][3]

The syntax of the usage has obviously changed (apart from "prefixing" with of, in, upon, etc., it used to be possible to follow it with that, than, etc., for example). But personally I can't see that the meaning has really changed much.

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).

¹ Odd/dated phrasing by OED. They mean because there's no [spoken] stress [on the syllable]. [1]: http://en.wikipedia.org/wiki/Etymological_fallacy [2]: https://www.google.com/search?q=%22go%20less%20you%20do%22&btnG=Search%20Books&tbm=bks&tbo=1&gws_rd=ssl [3]: https://www.google.com/search?q=%22You%20don%27t%20have%20to%20go%20less%20you%20want%20to%22&btnG=Search%20Books&tbm=bks&tbo=1&gws_rd=ssl

I don't really see how the etymological fallacy (the mistaken belief that the "true" or "proper" meaning of a word is its oldest or original meaning) relates to current usage of unless.

OED's "etymology" for the word says...

less ... with the prepositions of, in, upon, and on ; the last of these by want of stress¹ has been assimilated in form to the prefix un-

Semantically, the specific preposition is fairly irrelevant. Casual/dialectal speakers commonly discard it completely (I have no opinion on whether the apostrophe should be present)...

"I won't go 'less you do, Josh," said Lester.
"You don't have to go less you want to."

The syntax of the usage has obviously changed (apart from "prefixing" with of, in, upon, etc., it used to be possible to follow it with that, than, etc., for example). But personally I can't see that the meaning has really changed much.

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).

¹ Odd/dated phrasing by OED. They mean because there's no [spoken] stress [on the syllable].