Definition of lest conjunction from the Oxford Advanced Learner's Dictionary

     

    lest

     conjunction
    conjunction
    BrE BrE//lest//
     
    ; NAmE NAmE//lest//
     
    (formal or literary)
     
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  1. 1in order to prevent something from happening He gripped his brother's arm lest he be trampled by the mob. Lest anyone should doubt my story, I have brought documents to attest to its truth.
  2. 2used to introduce the reason for the particular emotion mentioned She was afraid lest she had revealed too much.
  3. Word Origin Old English thȳ lǣs the ‘whereby less that’, later the læste.
See the Oxford Advanced American Dictionary entry: lest