unless

Meanings

1. 1
   conj

   Except on a specified condition; if not.

   I’m leaving unless I get a pay rise.

2. 2
   conj

   If not; used with counterfactual conditionals.

   Who would know of Alexander or Caesar, or would respect the Stoics or the Peripatetics, unless they had been distinguished by the memorials of writers?

3. 3
   conj

   Except if; used with hypothetical conditionals.

   Q. If Lieutenant P. had given the word "Fire," would you have fired, and at what?—A. I should not have known, unless he had told me what to fire at.

Etymology

From Middle English unlesse, earlier on lesse (modern on + less), from (not) on lesse (than) (“(not) on a less compelling condition (than)”). The quality of negation in the word and the lack of stress changed on to un-.