Flat out means as hard as possible, as fast as possible, with the maximum effort possible. The term flat out is also used in North America to mean utterly, without equivocation, out-and-out, absolute. When used as an adjective before a noun, the term is hyphenated as in flat-out. The term flat out is known at least from the 1930s to describe going as fast and as hard as possible, probably derived from the act of depressing an automobile accelerator to the floor. In the 1830s in North America, flat out meant to fail utterly. How flat out came to mean absolutely and without equivocation in North America is unknown.
Examples
Green light: Hong Kong to go flat out to build eco-friendly skyscrapers (The South China Morning Post)
To be able to throw on the run with no pain, and to be able to just flat-out run in general has been really great so far.” (The Minneapolis Star-Tribune)
Lt. Gov. Brian Calley on Thursday said a Democratic National Committee video aired at Democrats’ national convention contained a “flat out lie” about presidential candidate Hillary Clinton’s role in getting state government to take action in Flint’s lead-contaminated water system. (The Detroit News)
Where I currently am most of the time, it’s flat out impossible to play. (Forbes Magazine)
We reserve the right to ban impersonators and remove comments that contain personal attacks, threats, or profanity, or are flat-out offensive. (Philadelphia Magazine)
Essentially, the runners spend the first 60 meters of the race reaching top speed, and then the next 140 meters cutting back just enough to hit their target 200-meter time, which is about one second slower than they can run a flat-out 200 meters. (New York Magazine)