'thin-beaked' originates from Modern English, specifically the combination of the adjective 'thin' and the noun 'beak' to form a descriptive compound.

'thin' comes from Old English 'þynne' (from Proto-Germanic *þunną) meaning 'thin'; 'beak' comes from Old North French 'bec' / Old French 'bec', ultimately from a Germanic source (compare Dutch 'bek') and was adopted into Middle English as 'bek'/'beik', later becoming 'beak'. The compound 'thin-beaked' was formed in Modern English by joining the two words.

Individually, 'thin' originally meant 'not thick' and 'beak' meant 'bill' (a bird's mouth part); combined as 'thin-beaked' the phrase straightforwardly came to mean 'having a thin beak' with little semantic shift beyond mere description.