it/that won't do

It/that is totally unacceptable or unsatisfactory. Primarily heard in UK. A: "It will probably be the end of next week before I'm finished with this project." B: "That simply won't do, Jenkins. I don't care how, but I want it finished by Monday morning!" I was going to tell them I couldn't come to dinner because of a prior engagement, but I knew that wouldn't do. No, that won't do. This woman is one of our best clients—you have to pick her up right away.

it/that (just) won’t ˈdo

it/that will never ˈdo

(especially British English) used to say that a situation is not satisfactory and should be changed or improved: He’s spending every afternoon in the park with his friends instead of going to school, and that just won’t do! ♢ I feel very upset but it would never do to show it.