1. pertaining to or having flat feet. A lady should never waltz if she feels dizzy. It is a sign of disease of the heart, and has brought on death. Neither should she step flat-footed, and make her partner carry her round; but must do her part of the work, and dance lightly and well, or not at all. (Mrs. John M. E. W. Sherwood , Manners and Social Usages)
2. off one’s guard; unprepared. Mining companies were caught flat-footed by cyber attacks.