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]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "unless":
   aside from, bar, barring, beside, besides, but, ex, except,
   except for, except that, excepting, excluding, exclusive of,
   if not, leaving out, let alone, omitting, outside of, precluding,
   save, save and except, saving, than, unless that, were it not,
   without