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
Farlex Dictionary of Idioms.

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
Farlex Partner Idioms Dictionary

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
See also:
  • and that's flat
  • And that's that
  • That’s that!
  • that's that
  • That's that!
  • (and) that's that
  • chip
  • chips
  • cut a dash
  • dead as mutton
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!"