diplomatic

1705–15; < French diplomatique < New Latin diplōmaticus, equivalent to Latin diplōmat- (stem of diplōma) diploma + -icus -ic

2. Diplomatic, politic, tactful imply ability to avoid offending others or hurting their feelings, especially in situations where this ability is important. Diplomatic suggests a smoothness and skill in handling others, usually in such a way as to attain one's own ends and yet avoid any unpleasantness or opposition: By diplomatic conduct he avoided antagonizing anyone. Politic emphasizes expediency or prudence in looking out for one's own interests, thus knowing how to treat people of different types and on different occasions: a truth which it is not politic to insist on. Tactful suggests a nice touch in the handling of delicate matters or situations, and, unlike the other two, often suggests a sincere desire not to hurt the feelings of others: a tactful way of correcting someone.

English diplomatic comes from New Latin diplōmaticus “pertaining to the science of deciphering old official documents ( diplōmata ) and determining their authenticity, age, etc.”
The Latin adjective is derived from diplōmat-, the inflectional stem of diplōma, which has several technical meanings: “an official letter of recommendation; a certificate or license granting a privilege; a folded tablet or paper carrying an official’s instructions to allow the bearer to have free passage and assistance.”
The journey from an adjective relating to the science of analyzing documents (which in English is known as diplomatics ) to one pertaining to the art of international relations ( diplomacy ) is a fascinating one. Díplōma is a derivative of the verb diploûn “to repeat, multiply by two, repay twofold.” Diploûn is a compound of the prefix di- “two, twice, double,” a variant of the adverb dís “twice.” The original Greek word díplōma from which the Latin diplōmaticus comes meant “a piece of paper folded in two,” and in Roman imperial times, “an order allowing the bearer to use the imperial transportation system; a passport.”
Diploma in English retains the sense of a piece of paper, one that we receive (often rolled, not folded) when we graduate, formally granting an academic degree. A less well-known sense of diploma is “a public or official document.” The earliest meanings of diplomatic in English referred first to the scientific analysis of these documents, then to the documents themselves, and, at a tempestuous historical moment, the word becomes forever associated with the activities surrounding the documents.
In the 18th century, French writer and historian Jean Dumont published Corps universel diplomatique du droit des gens, containing the original texts of the treaties of Alliance, of Peace, and of Commerce, from the Peace of Munster to 1709. This was a collection of documents as the title indicates, and the term corps diplomatique at first referred to the body of documents but over time underwent a meaning transfer to the subject matter of the documents themselves, international relations.
By 1789, we can find the term used in English but with the French spelling diplomatique. Edmund Burke, in his best-selling Reflections on the Revolution in France, popularizes the shift by using corps diplomatique in this novel way: “The Prussian ministers in foreign courts have … talked the most democratick language …. The whole corps diplomatique, with very few exceptions, leans that way.” Here, the term is applied to the ambassadors and officials making up the diplomatic body, that is, the group of ministers engaged in diplomacy and international relations (and not the body of documents collecting itself into piles to be examined). Soon after, the French spelling is dropped in favor of the Anglicized diplomatic, but the French connection remains.