Dreye權威釋義|unless

conj.

prep.

unless and until

直到……才
I will not believe it unless and until he tells me so.
除非他親口對我說，否則我絕不相信。

同義參見:

以上來源於：《英漢大辭典》

conj.

except when; if not.

ME: from on or in (assimilated through lack of stress to un-¹) + less.

以上來源於：《簡明牛津英語詞典》