What is the meaning and usage of 'Lest' in the following sentences:
Hold fast lest you should fall.
Lest anyone should think it strange,let me know what it is.
Lest anyone should think it's strange, let me assure you that it is quite true.
I looked up the meaning and usage of 'Lest' in http://dictionary.cambridge.org/dictionary/english/lest but I didn't understand it. Please make its meaning and usage it clear to me
Source: I found them In a book named 'Lovely English conversation practic' Written by chudamani Gautam