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.”