'flat-billed' is a Modern English compound formed from the adjective 'flat' and the noun 'bill' with the participial/adjectival suffix '-ed'. 'flat' originates from Old Norse, specifically the word 'flatr', where 'flatr' meant 'level' or 'broad'; 'bill' originates from Old English, specifically the word 'bile', where 'bile' meant 'beak' or 'bill'.

'flat-billed' developed in Modern English as the compound of 'flat' + 'bill' + '-ed'. 'flat' entered English via Old Norse 'flatr', and 'bill' comes from Old English 'bile' meaning 'beak'; over time the compound came to be used descriptively in zoology and ornithology to denote species with a flattened bill.

Initially the components literally described shape—'flat' (level, broad) + 'bill' (beak)—so the compound meant 'having a flat bill'; the meaning has remained largely literal and descriptive into modern usage, especially in scientific and natural-history contexts.