a. 1. Having a flat foot, with little or no arch of the instep; suffering from fallen arches. [ wns=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. [ wns=4 ]
Syn. -- downright, forthright, foursquare, head-on, straightforward. [ PJC ]

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

6. Unprepared and unable to react quickly; as, the new product caught their competitors flat-footed. [ wns=2 ]
Syn. -- unready. [ WordNet 1.5 ]

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