it/that won't do

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 will not 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.
See also: that

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.
See also: that