It/that won't do British spoken language phrase
Said when you mean that something should not be allowed or is not good.
The teacher caught me holding the book in the exam. I was about to tell her that I was just putting it back into my bag, but I knew that wouldn't do.
A; What do you think of friends with benefits. B: To me, it won't do.
He is trying to make excuses for secret affair. However, whatever reasons he gives, it just won't do.
The origin of this phrase is not clear.
Someone who is totally bad at doing something.
Mary really can't sing for toffee.