WordNet (r) 3.0 (2006):

flat out
    adv 1: in a blunt direct manner; "he spoke bluntly"; "he stated
           his opinion flat-out"; "he was criticized roundly" [syn:
           bluffly, bluntly, brusquely, flat out, roundly]
    2: at top speed; "he ran flat out to catch the bus"; "he was off
       down the road like blue murder" [syn: flat out, like blue
       murder]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "flat out":
   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