flat rate

(UK) Enough to fill a flat .

A British expression used to signal that a decision is final and the speaker will not entertain further argument — equivalent to 'and that's that' or 'end of discussion.' The word 'flat' here means definitive or absolute, the same sense found in 'flat refusal' or 'flat out.' Now largely dated, it was typically spoken with a dismissive, authoritative tone by someone asserting their position. You might still encounter it in older British fiction or from speakers of a certain generation.

(also, figurative) An unchanging state, as indicated in a graph of a variable over time.

(UK, dated, informal) And that's that.

A dated British informal phrase used to close a discussion or declare a final decision, equivalent to and that's that. The flat element means definitive and without further discussion. Signals the speaker has made up their mind and will not be moved. Somewhat old-fashioned in register now; more commonly encountered in older British writing and speech than in contemporary use.