Word to Learn, Remember and Use - "Lest"

What does lest mean and when do we use it?

Lest is a very rare word and quite old fashioned. 

Most people in Britain know it, because we see it written very often in the same place - on war memorials, on statues, which have been put up so that we remember people who died in wars; and what's very often written on these statues is ‘lest we forget’! Now, what lest means is ‘so that we don't’ or ‘so that you don't’. It's a warning. It's introducing a danger to be avoided. 

Another way of putting it would be "so as to avoid". It is just a faster way of saying it. if directed at a person it would mean something more like "so that you don't" 

Example: Wear gloves so as to avoid getting caustic chemicals on your hands. 

Using "Lest" Wear gloves lest you get caustic chemicals on your hands. 

Example: Put the parking brake on so as to avoid having to chase your car down the hill. 

Using "Lest": Put the parking brake on lest you have to chase your car down the hill. 

An archaic example: "He will bear you up lest you dash your foot upon a stone" meaning: He'll carry you so you don't cut your foot on a rock.