except under the circumstances that: I'll be there at nine, unless the train is late.
preposition
2.
except; but; save: Nothing will come of it, unless disaster.
Origin: 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 preposition and conjunction in the early 16th century; see on, less
a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison.
the offspring of a zebra and a donkey.
a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.
a stew of meat, vegetables, potatoes, etc.
a screen or mat covered with a dark material for shielding a camera lens from excess light or glare.
a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.
1467, earlier onlesse (c.1440), from on lesse (than) "on a less condition (than)," the first syllable originally on, but the negative connotation and the lack of stress changed it to un-.