"the science of formal intercourse between nations through authorized agents; the art of negotiating and drafting treaties;" more loosely, "transactions and management of international business in general," 1793, from French diplomatie, formed from diplomate "diplomat" (on model of aristocratie from aristocrate), from Modern Latin diplomaticus (1680s), from Latin diploma (genitive diplomatis) "official document conferring a privilege" (see diploma; for sense evolution, see diplomatic).
It is obvious to any one who has been in charge of the interests of his country abroad that the day secrecy is abolished negotiations of any kind will become impossible. [Jules Cambon, "The Diplomatist" (transl. Christopher Rede Turner), 1931]
Meaning "dexterity or skill in managing negotiations of any kind" is by 1848.