Meaning of "it will not do to risk a fainting fit."

Question

Below is a passage from Jane Eyre:

‘You don’t turn sick at the sight of blood?’

‘I think I shall not: I have never been tried yet.’

I felt a thrill while I answered him; but no coldness, and no faintness.

‘Just give me your hand,’ he said: ‘it will not do to risk a fainting fit.’

Those are Mr Rochester and Jane Eyre speaking. I'm struggling to understand the meaning of the words in bold. "It will not do to [verb]" seems to be a collocation, but I can't seem to find its meaning. All I've found are expressions like "That'll do (fine)" or "That will not do!", but those seem unrelated (or I can't see the relation, anyway).

What does Mr Rochester mean by ‘it will not do to risk a fainting fit’?

Also, similar sentences to see how this collocation works would be much appreciated.

Answer 1

One of the many meanings of do is "be appropriate, be suitable".

It won't do to X or X won't do is an idiomatic way of saying "X is not appropriate/suitable/satisfactory".

So yes, actually "That will do fine" or "That will not do!" is in fact related - it just means "That will be perfectly acceptable" or "That will not be suitable!" Some examples of ways in which you might use this kind of phrase:

It won't do to be seen in those old clothes. (that is, it won't be appropriate to be seen in those old clothes)

I see you have some rubber boots; those will do for our fishing trip. (that is, those boots will be suitable)

How much money do you have on you? Twenty dollars? That'll do. (that is, twenty dollars is sufficient)

Answer 2

it will not do

means the same as "it is unacceptable":

‘Just give me your hand,’ he said: ‘it is unacceptable to risk a fainting fit.’