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The Collaborative International Dictionary of English v.0.48 (gcide)

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]