- (idiomatic) At top speed.
- After 10 minutes of running flat out, he was out of breath.
- (idiomatic) bluntly, no holds barred
- She thought it was best to tell him she didn't love him flat out.
- (idiomatic) lazy, sleeping (Australian)
- flat out (like a lizard in the sun) - doing absolutely nothing
a outrance, a toute outrance, absolute, absolutely, admitting no exception, all hollow, all out, all the way, all-out, apace, at full blast, at full drive, at full speed, at full throttle, beyond all bounds, beyond compare, beyond comparison, beyond measure, blasted, categorical, clear, complete, completely, conclusive, damned, dead, decided, decisive, definite, definitive, determinate, downright, entire, essentially, expeditiously, explicit, express, extremely, final, fixed, flat, flat-out, full speed ahead, fundamentally, global, hastily, immeasurably, implicit, in excess, in full sail, in the extreme, inappealable, incalculably, indefinitely, indisputable, infinitely, lickety-split, most, out-and-out, outright, peremptory, perfect, perfectly, positive, posthaste, purely, quickly, radically, rank, rapidly, round, speedily, straight, straight-out, swiftly, to a fare-you-well, to a fault, to excess, to extremes, to the backbone, to the extreme, to the full, to the limit, to the marrow, to the skies, to the sky, to the utmost, too far, total, totally, uncircumscribed, unconditional, unconditionally, unconditioned, under full steam, undoubting, unequivocal, unequivocally, unhampered, unhesitating, unlimited, unmistakable, unmitigated, unqualified, unquestioning, unreserved, unrestricted, unwaivable, utter, utterly, whole, wide open, with a vengeance, with all speed, without exception, without reserve