and that's flat!

and that's flat

old-fashioned 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. Look, I told you I'm moving to Canada after high school, and that's flat! Listen, missy, you're grounded for a week, and that's flat!
See also: and, flat
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.

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
Farlex Partner Idioms Dictionary © Farlex 2017
See also:
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!"