Post History

It is generally easier to track down the earliest usages of a word, than the earliest usages of an entire phrase from which the word eventually developed.  I'll offer two speculative answers; they are not mutually exclusive, because we are discussing developments over a vast time and space.

1. In Middle English, the original meaning appears to have been a litotes.  Instead of a literal "...unless X occurs", the phrase rather literally meant something like "unless anything at least as absolving as X occurs".  You can equate "[on not] less of a reason than X" with "[on] at least as good a reason as X" here.  However, either could have been a habitual litotes for simply "X" even in case of the original longer phrase.


2. Someone might say finish their promise even in Modern English, made for tomorrow, with "unless the world ends tonight" or "unless I change my mind"; or end their factual statement with "unless I'm very mistaken".  Such usage isn't attaching any specific conditions to the claim.  It's rather used to indicate its general strength.  Perhaps the modern core meaning of "unless" has evolved from mediaeval usage indicating the strength of a claim by naming an important example of its limits (whatever the perceived limits happened to be).  Perhaps both those meanings were available in the past, and both are still available today; and only the centre of gravity of the meaning of the phrase has shifted, along with the phrase evolving to a single word.