'flat-footed' originates from English, combining the adjective 'flat' (from Old English 'flǣt') and the noun 'foot' (from Old English 'fōt'), with the adjectival suffix '-ed' added to form an adjective.

'flat' and 'foot' are Old English roots ('flǣt' and 'fōt'); the compound 'flat-footed' is attested in Modern English by the 19th century meaning 'having flat feet' and later used figuratively to mean 'slow or unprepared'.

Initially it meant 'having flat feet' (a literal, physical condition), but over time it developed a figurative meaning of 'slow to react, clumsy, or unprepared.'