Webster 1913 Edition
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.
unlesswe 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
unlessthe nightly owl.
Webster 1828 Edition
Except; that is, remove or dismiss the fact or thing stated in the sentence or clause which follows. 'We cannot thrive unless we are industrious and frugal.' the sense will be more obvious with the clauses of the sentence inverted. Unless, [remove this fact, suppose it not to exist,] we are industrious and frugal, we cannot thrive. Unless then answers for a negation. If we are not industrious, we cannot thrive.