/ lest; NAmE / conjunction ( formal or literary )
1.
in order to prevent sth from happening :
He gripped his brother's arm lest he be trampled by the mob.
2.
used to introduce the reason for the particular emotion mentioned
SYN in case :
She was afraid lest she had revealed too much.
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WORD ORIGIN
Old English thӯ lǣs the whereby less that , later the læste .