I made them a reasonable offer but they turned it down flat.

我向他们提出一个合理建议，但是他们断然拒绝了。

《牛津词典》

He turned a person of suggestion down flat.

他断然拒绝那个人的建议。

youdao

Use a pan big enough to lay each piece of fish down flat.

要使用足够大的锅，要让每一块鱼肉都可以平放在锅底上。

youdao

One was in the second row and the other four bodies were found "laying down flat in the cargo area, " Babeu said.

CNN: 5 bodies found in burned-out SUV in Arizona desert

The law firm was turned down flat by the government's trademark bureau.

ECONOMIST: China's powerful bureaucrats have lost a lawsuit

Mr. Gates turned Mr. Hoekstra down flat.

CENTERFORSECURITYPOLICY: Let them meet the neighbors