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