mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less. The first syllable originally on, but the negative connotation and the lack of stress changed it to un-.
à moins que = sauf si.
a menos que means 'unless' in Spanish and Portuguese.
a meno che means 'unless' in Italian.
The above substantiates the same semantic shift in at least 4 languages,
from the prepositional phrase
- a/à/un ← on (preposition) +
- less (adverb) +
- than (comparative preposition)
that semantically shifted to mean 'if not'.
What type from the Typology by Blank (1999) most fits 'unless'?