Is ‘lest’ even a word that can be used?

Sometimes I see “lest” appear in client’s writing. Often it’s followed by a note asking, “Is this even a word?” It is indeed a word and acceptable to use. “Lest” means “so as not to” as in “The burglar stepped quietly lest he be heard.”

Whenever using “lest”, don’t put “not” after it, as in as in “The burglar stepped quietly lest not he be heard”, or you’ll end up with a sort of double negative – your sentence then would be the equivalent of saying, “The burglar stepped quietly so as not to be heard.”

As “lest” is rarely used in modern English, many readers will find it an odd word, so I recommend seeing if you can rewrite the sentence to avoid its use.