'flat-surfaced' originates from Modern English as a compound of 'flat' (Old Norse/Old English origin) and 'surface' (from Old French and Latin 'superficies'), with the adjectival/past-participle suffix '-ed'

'flat' comes from Old Norse 'flatr' meaning 'broad, flat', and 'surface' comes from Old French 'surface' (from Latin 'superficies', from 'super-' 'above' + 'facies' 'face, form'); these elements were combined in Modern English to form compounds like 'flat-surfaced' to describe objects having a flat surface

Initially the components referred separately to 'flat' and 'surface'; over time the compound came to be used as an adjective meaning 'having a flat surface' and is used descriptively in Modern English