Unprepared and unable to react quickly; as, the new product caught their competitors flat-footed.
having broad flat feet that usually turn outward; "a slow flat-footed walk"
To go flat-footed, is spoken of an airman without an aeroplane, or a member of the tank corps travelling on foot.
Condition A character who has not yet acted during a combat is flat-footed, not yet reacting normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.