in order to prevent something from happening 免得；以免 He gripped his brother's arm lest he be trampled by the mob. 他紧抓着他弟弟的胳膊，怕他让暴民踩着。 Lest anyone should doubt my story, I have brought documents to attest to its truth. 以免有人怀疑我的故事，我带来了文件来证明它的真实性。
used to introduce the reason for the particular emotion mentioned （引出产生某种情感的原因）唯恐，担心
Word OriginOld English thȳ lǣs the ‘whereby less that’, later the læste.