The Collaborative International Dictionary of English v.0.48:

Flat-footed \Flat"-foot`ed\, a.
   1. Having a flat foot, with little or no arch of the instep;
      suffering from fallen arches. [WordNet sense 3]
      [1913 Webster]

   2. Firm-footed; determined. [Slang, U.S.]
      [1913 Webster]

   3. clumsy; amateurish; pedestrian; unimaginative; plodding;
      as, flatfooted prose.
      [PJC]

   4. Without reservation; without evasion or compromise; firm;
      as, a flat-footed refusal; a flatfooted denial. [WordNet
      sense 4]

   Syn: downright, forthright, foursquare, head-on,
        straightforward.
        [PJC]

   5. With feet flat on the ground; not tiptoe. [WordNet sense
      1]
      [WordNet 1.5]

   6. Unprepared and unable to react quickly; as, the new
      product caught their competitors flat-footed. [WordNet
      sense 2]

   Syn: unready.
        [WordNet 1.5]

   To catch (one) flatfooted to catch (a person) unprepared;
      to catch (a person) by surprise.
      [PJC]