Lest is a conjunction that means "in order to prevent something from happening". It introduces the reason for a particular emotion or is used in 'purpose' clauses to mean "for fear that". Lest can be used to say "She was afraid lest she had revealed too much" meaning "in case she had revealed too much". It also expresses purpose, like in "He dared not spend the money lest someone should ask where he had got it", meaning "for fear that someone should ask where he had got it".

1. Usage of “LEST’.
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Nageswar Rao. A
English Teacher.
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Sri. K. V. Madhusudhan
Resource Person (English)

2. Lest is a conjunction. It gives the meaning ‘in order to prevent
something from happening’.
He gripped his friend’s arm lest he be slipped from the slope.
It is used to introduce the reason for the particular emotion
mentioned.
She was afraid lest she had revealed too much. (here it gives
the meaning of ‘in case’)

3. ‘Lest’ is also used to express ‘purpose’ clauses to mean ‘for
fear that’
He dared not spend the money lest someone should ask
where he had got it.
In literary English ‘lest . . . Should’ is sometimes placed
after expressions of fear or anxiety.
He was terrified lest he should slip on the icy rocks.

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