'slender-beaked' is a modern English compound formed from 'slender' + 'beaked', where 'slender' meant 'thin or slight' and 'beak' meant 'bill' (with '-ed' forming the adjective 'beaked').

'slender' comes from Middle English 'slendre'/'slender' (of uncertain origin, possibly influenced by Old Norse), while 'beak' comes from Old French 'bec' (from Late Latin 'beccus'); these elements combined in modern English to form descriptive compounds such as 'slender-beaked'.

Initially the separate words meant 'thin' and 'bill' respectively; combined as a compound they retain the descriptive sense 'having a thin beak' in modern usage.