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From The Collaborative International Dictionary of English v.0.48:
Unless \Un*less"\, conj. [Formerly, onles, onlesse, onlesse that, that is, in less, in a less case. See On, and Less.] Upon any less condition than (the fact or thing stated in the sentence or clause which follows); if not; supposing that not; if it be not; were it not that; except; as, we shall fail unless we are industrious. [1913 Webster] Note: By the omission of the verb in the dependent clause, unless was frequently used prepositionally, -- a construction common in Shakespeare and still employed colloquially. [1913 Webster] Here nothing breeds unless the nightly owl. --Shak. [1913 Webster]