WordWeight : Weighing the Words

Pronunciation : Un*less"
Part of Speech : conj.
Etymology : [Formerly, onles, onlesse, onlesse that, that is, in less, in a less case. See On, and Less.]
Definition : Defn: 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.

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. Here nothing breeds unless the nightly owl. Shak.
Source : Webster's Unabridged Dictionary, 1913