Source : Webster's Revised Unabridged Dictionary (1913)

Flat-footed \Flat"-foot`ed\, a.
   1. Having a flat foot, with little or no arch of the instep.

   2. Firm-footed; determined. [Slang, U.S.]

Source : WordNet®

flat-footed
     adj 1: with feet flat on the ground; not tiptoe
     2: unprepared and unable to react quickly; "the new product
        caught their competitors flat-footed"
     3: having broad flat feet that usually turn outward; "a slow
        flat-footed walk"
     4: without reservation; "a flat-footed refusal"