'flat-toed' originates from Modern English, specifically the compound of 'flat' + 'toed', where 'flat' ultimately comes from Old Norse 'flatr' meaning 'broad, level' and 'toe' comes from Old English 'tā' meaning 'toe'.

'flat' entered English from Old Norse 'flatr' and developed in Middle English as 'flat'; 'toe' comes from Old English 'tā'. The compound 'flat-toed' is a straightforward Modern English descriptive compound formed from these two elements.

Initially it literally described the physical quality 'flat' + 'toe' (toes that are flat); over time it has remained a literal descriptive term without major semantic shift.