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.