it/that will never do

it/that will never 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'll never 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 would never do.
See also: never, that, will