lest this should undo

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I've just read some threads about "lest", but I cannot understand the meaning of this word in the following sentence.

But he should avoid cohabitation that day and night lest this should undo all that we have spoken of, which is most beneficial for health...

I think it means (please tell me if I'm wrong)

That day and night he shouldn't have sex (cohabitate). But if you have sex that day, then you shouldn't follow my abovementioned advices (which are beneficial for health...).
  • Franco-filly

    Senior Member
    English - Southern England
    "lest it should undo" means "for fear that it should undo" In other words "unless you want to undo / because it could undo all that we have spoken about, which (i.e. the thing we have spoken about) is most beneficial for health.."


    Senior Member
    UK English
    Another way of understanding it is to replace lest with in case.

    Example: Keep your voice down lest you wake the baby. (A rather formal way of saying in case you wake the baby.)
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