for fear that; so that (one) should not (used negatively to introduce a clause expressive of an action or occurrence requiring caution): He kept his notes by his side lest faulty memory lead him astray.
that (used after words expressing fear, danger, etc.): There was danger lest the plan become known.

before 1000; Middle English leste, contraction of the lesse the, thi les the; late Old English the lǣste, earlier thȳ lǣs the, literally, whereby less that (thȳ instrumental case of the demonstrative and relative pronoun, lǣs less, the relative particle)

least, lest, let's.
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World English Dictionary
lest (lɛst)
1.  so as to prevent any possibility that: he fled the country lest he be captured and imprisoned
2.  (after verbs or phrases expressing fear, worry, anxiety, etc) for fear that; in case: he was alarmed lest she should find out
[Old English the lǣste, earlier thӯ lǣs the, literally: whereby less that]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

c.1240, from O.E. phrase þy læs þe "whereby less that," from þy, instrumental case of demonstrative article þæt "that" + læs (see less) + þe "the." The þy was dropped and the other two words contracted into leste.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The designer won't discuss her plans in more detail, lest she divulge trade
Also, be sure to stay off the bikers' lane, lest you get clipped or cursed at
  by speeding cyclists.
At the same time, a welcoming remark should be serious, lest you suggest that
  the occasion is not a solemn one.
Lest you think my rhetoric too strong, look at what the great recession has
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