'flattened-toed' originates from Modern English, specifically the words 'flattened' and 'toe', where 'flattened' is the past participle of 'flatten' meaning 'made flat' and 'toe' meant 'digit of the foot'.

'flattened' developed from Middle English 'flatten' (from adjective 'flat' + verbal -en), and 'toe' comes from Old English 'tā' (digit). These elements were later combined in modern English as a descriptive compound 'flattened-toed'.

Initially, 'flatten' meant 'to make flat' and 'toe' simply meant a foot digit; over time the compound came to be used adjectivally to describe the condition or adaptation 'having flat toes'.