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.
⇒ By the omission of the verb in the dependent clause, unless was frequently used prepositionally, -- a construction common in Shakespeare and still employed colloquially.
Here nothing breeds unless the nightly owl.
Shak.
© Webster 1913.