1. For fear that: tiptoed lest the guard should hear her; anxious lest he become ill.
2. So that someone or something not; in case someone or something might: "Lest anyone forget, adolescence is purgatory" (Jon Pareles).
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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