Unless, conj.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for unless; he lists it under but.
Where he writes about it:
So may the use of but in the sense of unless; as, "it never rains but it pours." To the same head must be referred the conditional use; as, "you may go, but with your father's consent" (i. e., "provided you have," "except that you must have," etc.). — from but