Why is the ninth month called September?
1711, "pertaining to documents, texts, charters," from Medieval Latin diplomaticus, from diplomat-, stem of diploma (see diploma).
Meaning "pertaining to international relations" is recorded from 1787, apparently a sense evolved in 18c. from the use of diplomaticus in Modern Latin titles of collections of international treaties, etc., in which the word referred to the "texts" but came to be felt as meaning "pertaining to international relations." In the general sense of "tactful and adroit," it dates from 1826. Related: Diplomatically.