A lexicon of words and expressions I have heard in the seven months being in this odd profession, which stick in my mind.
Cable: The basic form of communication between embassies and home. Traditionally messages were manually encoded and sent by telegram, so unless they liked spending hours on end squinting over a code book in dim candlelight diplomats tried to keep their cablegrams short and succinct. Now many countries use technology which bypass telegraph lines altogether.
Grateful: A taut but polite way to make a request is to start a sentence with this word, e.g.: Grateful post cables report by COB Friday. A friend and former diplomat has on his answering machine: Grateful you leave your name and number after the tone.
Optics: How a situation looks. For example, the optics of getting drunk and disorderly at a party would not be very favourable.
To Die in a Ditch (for something): To be overly committed to a particular position. This inelegant phrase was coined by Shakespeare in Antony and Cleopatra, but remains timeless despite its hackneyed useage.
Likeminded: Used as a noun to identify a country or person with similar views or imperatives - one of us.
Chapeau: The introduction in a treaty text that outlines its background and describes, sometimes with quaintly elevating language, what the objectives are.
Atmospherics: How a situation feels, particuarly when two sides are facing each other off. Other people might call it the 'under-current', the 'buzz' or just 'something in the air'.
Interlocutor: According to Wikipedia it describes someone who informally explains the views of a government and also can relay messages back to a government. Wikipedia adds it could colloquially refer to simply someone taking part in a conversation, but I have never heard of a five syllable colloquial word.
States Parties: A country that is a member of a treaty is a 'state party', although I say this with a disclaimer as there are a myriad of legal definitions on what correctly can be considered a country, a member and a treaty. The plural of 'state party' is neither 'state parties' nor 'states party', but 'states parties'. Beats me why - one theory is that it is a comprimise the Anglosphere had with the French: use English vocubulary with French grammar.
Heavy Lifting: The act of working hard to achieve something, even if in diplomacy the only physically arduous activity is remaining alert and well presented while the alcohol, caffeine and jet lag eat you from the inside.
I avail myself of this opportunity to renew to you the assurance of my highest consideration: A polite way to end a letter, reminding the readers that you are not taking them for granted.