плоскостопный означает в английский

flatfoot [flatfeet]▼(person)
noun
[UK: ˈflætfʊt] [US: ˈflætfʊt]

flat-footed▼(having flat feet)
adjective
[UK: flæt ˈfʊ.tɪd] [US: ˈflæt ˈfʊ.təd]

plantigrade▼(walking with the entire sole of the foot)
adjective
[UK: ˈplæn.tɪ.ɡreɪd] [US: ˈplæn.tɪ.ˌɡreɪd]