/un les", euhn-/ , conj.
1. except under the circumstances that: I'll be there at nine, unless the train is late.
prep.
2. except; but; save: Nothing will come of it, unless disaster.
[ 1400-50; late ME prepositional phrase on less (also o less ( e ), earlier upon less ) on a lesser footing or condition (than); first used as a prep. and conj. in the early 16th century; see ON, LESS ]