\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.

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.