Your Highness
Another one is ('), which contains 35 letters. "[It] is an adjective in the bureaucratic language of the 19th century meaning a very polite form of addressing clerks, something like Your Excellency, Your Highness, Your Majesty all together" (Guinness World Records 2003). Its dative singular form, (', with 36 letters) can be an example of excessively official vocabulary of the 19th century.