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.