UNLESS
Etymology
onlesse, from on lesse, from "on a less condition" ("on" became un-).
Conjunction
unless
Except on a specified condition; if not.
If not; used with counterfactual conditionals.
Except if; used with hypothetical conditionals.
Antonyms
• (except on a condition): if
Source: Wiktionary
Un*less", conj. Etym: [Formerly, onles, onlesse, onlesse that, that
is, in less, in a less case. See On, and Less.]
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition