Diplomatic
early 18th century (in the sense ‘relating to official documents’): from modern Latin diplomaticus and French diplomatique, from Latin diploma (see diploma). diplomatic (sense 1) (late 18th century) is probably due to the publication of the Codex Juris Gentium Diplomaticus (1695), a collection of originals of important public documents, many of which dealt with international affairs.
wiktionary
From French diplomatique, equal to diplomat + -ic.
etymonline
diplomatic (adj.)
1711, "pertaining to official or original documents, texts, or charters," from Modernl Latin diplomaticus (1680s), from diplomat-, stem of Latin diploma "a state letter of recommendation," given to persons travelling to the provinces, "a document drawn up by a magistrate," from Greek diploma "a licence, a chart," originally "paper folded double," from diploun "to double, fold over," from diploos "double" (see diplo-) + -oma, suffix forming neuter nouns and nouns that indicate result of verbal action (see -oma).
Meaning "pertaining to or of the nature of diplomacy; concerned with the management of international relations" is recorded by 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, skilled in negotiation or intercourse of any kind" it dates from 1826. Diplomatic immunity is attested by 1849. Related: Diplomatically.