An example of unless is saying you're going skiing this weekend, except if it doesn't snow - "I'll go skiing unless it doesn't snow."
Origin of unlessMiddle English onlesse, earlier on lesse that, in lesse that, at less than, for less
Origin of unlessMiddle English unlesse alteration ( influenced by un- not ) of onlesse on on ; see on . lesse less ; see less .
- (except on a condition): if
onlesse, from on lesse, from "on a less condition" ("on" became un-)