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1. As fast as possible. Even if I ran flat out, there's no way I could catch her—she's the fastest runner in the heat.
2. Bluntly or frankly. She flat out told us no, so I wouldn't ask her again.
1. clearly and definitely; holding nothing back. I told her flat out that I didn't like her. They reported flat out that the operation was a failure.
2. at top speed. How fast will this car go flat out? This car will hit about 110 miles per hour flat out.
1. In a direct manner, bluntly. For example, He told the true story flat out. [Colloquial; mid-1900s]
2. At top speed, as in She was running flat out to catch the train. [Slang; c. 1930]
flat out1 as fast or as hard as possible. informal 2 without hesitation or reservation; unequivocally. chiefly North American
1 1995 Independent Since August 1993 she has been working flat out on her latest three part documentary.
2 1993 Coloradoan She flat out said she didn't trust her fellow board members.
1 as fast as possible; with all the energy, strength, etc. you have: If I worked flat out, I could get all the repairs done today.
2 lying down, especially because you are ill or extremely tired: He was flat out on the bed.
3 (especially American English) in a definite and direct way; completely: I told him flat out ‘No’. ♢ It’s a 30-year mortgage, which we just flat out can’t handle.
1. mod. totally. We were all flat out disgusted.
2. mod. at top speed. If we run flat out, we can get there before dusk.
Thoroughly, outright; also, using all one’s resources, at full speed. An early recorded use of this adverbial phrase in 1932 (according to the OED) was in connection with automobile racing—that is, “driving flat out,” meaning as fast as possible. Presumably it alluded to a straight and level run of track, allowing full speed. This sense was soon transferred in such locutions as “They worked flat out to get the job done in time.” In succeeding decades the expression began to be used adjectivally in the sense of outright, as in “His last book was a flat-out failure” or “That’s a flat-out lie.”