unless Meaning, Definition & Usage

Un*less" conjunction
Etymology
Formerly, onles, onlesse, onlesse that, that is, in less, in a less case. See On, and Less.
Definitions
  1. 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