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flat out

B1 informal intransitive
In simple words

Going as fast or working as hard as you possibly can, or being so tired you can barely move.

Literal meaning: Lying flat with all air out — suggesting complete exhaustion or maximum effort with no reserve.

Meanings

1 B1 idiomatic informal

At maximum speed or with total effort.

"We were working flat out all weekend to finish the project on time."

"We've been going flat out since January."

— Common idiomatic usage in British journalism; cited widely in The Guardian and BBC news reports.
2 B1 idiomatic informal

Completely exhausted; unable to do anything more.

"After the marathon, she was flat out on the sofa for the rest of the day."

3 B2 idiomatic informal

Directly and without hesitation; plainly (used as an adverb).

"He flat out refused to apologize for what he had said."

"He flat out lied to Congress."

— Common political commentary phrase; widely used in US news media during congressional hearings.
Usage notes

Used widely in British and American English. As an adverb ('flat out lying'), it means completely or plainly. As an adjective ('I'm flat out'), it means exhausted or very busy. Tone is energetic or emphatic.

Commonly used with

run work lie deny sprint refuse

Forms

Base
flat out
I/you/we/they
3rd person
flats out
he/she/it
Past simple
flated out
yesterday
Past participle
flated out
have + pp
-ing form
flating out
continuous

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