The Collaborative International Dictionary of English v.0.48:

flat foot \flat" foot`\, flatfoot \flat"foot`\ (Med.)
   A foot in which the arch of the instep is flattened so that
   the entire sole of the foot rests upon the ground; also, the
   deformity, usually congential, exhibited by such a foot;
   splayfoot.
   [1913 Webster]