in a position in which the surface of something is against another surface without curving or sloping:
The bed can be folded flat for storage.
He lay flat on the floor.
That night I lay flat on my back and stared up at the ceiling.
in exactly three minutes, ten seconds etc - used to emphasize that something happens or is done very quickly:
I was dressed in five minutes flat.
if a joke, story etc falls flat, it does not achieve the effect that is intended:
Unfortunately, what could have been a powerful drama fell flat.
if you sing or play music flat, you sing or play slightly lower than the correct note so that the sound is unpleasant [≠ sharp]
to fall so that you are lying on your chest on the ground:
Babe slipped and fell flat on her face.
to not have the result you want or expect, especially when this is embarrassing:
The theory falls flat on its face when put into practice.
as fast as possible:
Everyone's working flat out to finish on time.
b) American English
in a direct and complete way [= straight out]
ask/tell somebody flat out
She asked him flat out if he was seeing another woman.
7 British English spoken
to tell someone something directly and definitely [= straight out]:
I told him flat that I didn't want to see him again.