Definition from macmillandictionary.com
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Carole says she’s not comfortable lying flat.
We heard a bang and threw ourselves flat on the ground.
He laid the map out flat on the table.
Brenda has always sung terribly flat.
I fell asleep in five seconds flat!
They turned me down flat.
We were flat broke at the time.
I warned you not to do it but you went flat against my advice.