Flat-footed

Flat·-foot·ed

Having a flat foot, with little or no arch of the instep; suffering from fallen arches.
Firm-footed; determined. [Slang, U.S.]
clumsy; amateurish; pedestrian; unimaginative; plodding; as, flatfooted prose.
Without reservation; without evasion or compromise; firm; as, a flat-footed refusal; a flatfooted denial.
With feet flat on the ground; not tiptoe.
Unprepared and unable to react quickly; as, the new product caught their competitors flat-footed.

To catch (one) flatfooted: to catch (a person) unprepared; to catch (a person) by surprise.