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-)