Definition of lest conjunction from the Oxford Advanced American Dictionary

     

    lest

     conjunction
    conjunction
    NAmE//lɛst//
     
    (formal or literary)
     
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  1. 1 in order to prevent something from happening He gripped his brother's arm lest he be trampled by the mob. Lest anyone doubt my story, I have brought documents to attest to its truth.
  2. 2used to introduce the reason for the particular emotion mentioned synonym in case She was afraid lest she had revealed too much.
See the Oxford Advanced Learner's Dictionary entry: lest