unless
UK: ʌnˈlɛs | US: ʌnˈlɛs
conj. except if; used to introduce the case in which a statement being made is not true or valid.
The word "unless" originated in late Middle English as a contraction of the phrase "on less than," where "on" meant "in the circumstance of" and "less than" implied a condition not being met. Over time, "on less" merged into "unless," with "un-" (a negating prefix) and "less" (from Old English "lǣssa," meaning "smaller/fewer") combining to convey the sense of "except if" or "if not." The logic reflects a conditional exclusion—something will not happen unless a specific condition is fulfilled.
You won’t pass the exam unless you study.
I won’t go to the party unless you come with me.
Unless it rains, we’ll have a picnic tomorrow.
The plant will die unless you water it regularly.
Don’t call me unless it’s an emergency.