and that's flat!

and that's flat

And that is final; and the issue is settled or completed. Primarily heard in UK. You are going to your sister's recital and that's flat! No more complaining.
See also: and, flat

and that's flat!

old-fashioned And that (decision) is final! Look, I told you I'm moving to Canada after high school, and that's flat!
See also: and

that's flat

used to indicate that you have reached a decision and will not be persuaded to change your mind. informal
See also: flat

and ˌthat’s ˈflat!

(British English, spoken) that is my final decision and I will not change my mind: I’m not lending you any more money, and that’s flat!
See also: and
References in classic literature ?
I do not wish to gloss over any fault the prisoner may have committed, if such a fault exists; but we owe her some consideration, and that's flat!"