'thin-billed' originates from English, specifically the words 'thin' and 'bill', where 'thin' meant 'having little thickness' and 'bill' meant 'beak'.

'thin' derives from Old English 'þynne' (from Proto-Germanic *þunnaz) meaning 'thin', and 'bill' (in the sense 'beak') comes from Old English 'bile'; the compound 'thin-billed' developed in Modern English as a descriptive compound for birds.

Initially, 'thin' meant 'having little thickness' and 'bill' meant 'beak'; over time the compound came to be used specifically to describe birds or animals 'having a thin beak'.