When Theresa May appointed legendary funnyman Boris Johnson to the post of Foreign Secretary in July of 2016, it must have seemed like a smart move.

In the Foreign and Commonwealth Office, the thinking went, he'd spend much of the time travelling the world, and therefore wouldn't be conspiring in Westminster. For the most part, foreign policy has broadly been a cross-party consensus for decades – he'd effectively be a glorified civil servant, making calls to gently urge foreign governments to chill when things get a bit heated, and pulling serious faces to talk about human rights, while simultaneously selling weapons systems to dictators. In other words, the political upshot for May is that it means fewer opportunities for grandstanding and point-scoring. It'd keep him out of trouble.

The problem is, for all of the classical references and knowing winks, jokester Boris simply can't control himself.

Since ziplining into the FCO, he has suggested that the Libyan city of Sirte only has to "clear the dead bodies" in order to become the next Dubai. And on an official visit to Burma, he thought it wise to recite a colonial-era Kipling poem while visiting one of the country's holiest sites. "Not appropriate," whispered the British Ambassador, who was luckily standing next to him at the time. These incidents are just some of the latest: Boris has made a self-described "thesaurus" of insults to various people and places around the world.

Still – perhaps this is funny old Boris, right? Surely everyone knows Boris is a joker. It's just typical, idiosyncratic Boris being fun. What a character! It's just banter. Right?

No. Not being able to control your language is serious – and his most recent "slip of the tongue" is just the latest example as to why.

Speaking to the Commons Foreign Affairs Select Committee, he inadvertently made things worse for British-Iranian dual citizen Nazanin Zaghari-Ratcliffe, who is currently imprisoned in Iran for supposedly plotting against the government.

While the official line is that she was only in the country visiting family, Boris told the Select Committee that she was teaching journalism – which in theocratic Iran is considered a crime against the state. As the BBC reports, since Boris said this, Zaghari-Ratcliffe has been summoned back to court – and his words have been cited as new evidence against her, which could see her sentence doubled.

Whoops! Gosh! What japes! Suddenly Boris doesn't seem quite so fun any more.

But this disturbing incident is merely symptomatic of a larger problem, which shows that Boris is manifestly unsuited to the role of Britain's Chief Diplomat.

In international diplomacy, words matter. They shape how we see the world – and the use of particular words or phrases can cause instability, or help make peace. The world, it turns out, is a linguistic minefield.

Should the Armenian Genocide (which took place during WWI) be called a "genocide"? Not according to Turkey, which will have a very hostile reaction if you do otherwise, even though it took place a century ago.

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Every Eurovision Song Contest has to painstakingly refer not to "Macedonia" but to the "Former Yugoslav Republic of Macedonia" in order to placate Greece, which is worried that recognising the name would legitimate Macedonia's territorial claims over parts of Northern Greece, which is where there was an ancient kingdom also called Macedonia.

And people on different sides of the Ukraine conflict will judge you based on whether you refer to "The Ukraine" or just "Ukraine" – the former being how Russians referred to the country during the Soviet era, and the latter being the preferred name since 1991, as it doesn't carry any colonial baggage.

Taiwan is perhaps the starkest example of why this matters. For all practical purposes, it is an independent country to China. It has its own government, it participates in the Olympics as a separate country and so on. But if you refer to it as though it was not a part of China? Well, there'd be hell to pay.

To get over this, governments around the world all carefully choose their words in order to participate in the fiction that Taiwan is part of China… even when it isn't, in any practical sense. If Britain, and any other countries, want to do business with the People's Republic of China, it needs to choose its language carefully.

So it's easy to see why a Minister who doesn't think before he opens his mouth might be a bit of a risk.

Perhaps, in the case of Zaghari-Ratcliffe, Boris is absolutely right. Perhaps she was teaching journalism? Imagine, for the sake of argument, that she was. Hell, that'd be a brave and admirable thing to do in a country which lacks many of the freedoms that we take for granted in the west. But it would also serve the greater diplomatic good, and would help Nazanin Zaghari-Ratcliffe more, if we all maintained the fiction that she was just visiting family.

Now that Boris has waded in, and has inadvertently let the cat out of the bag, it basically obliges Iran to double down and act tougher on the traitor – and it damages relations with Britain because now it looks like we're supporting someone who, in the eyes of the Iranian regime, is working to overthrow the government. Increased tension all round, and a double prison sentence for someone who shouldn't be imprisoned at all.

In effect, Boris is doing the exact opposite of diplomacy. By not controlling himself – seemingly something he is incapable of doing – he is making things worse. It's... almost like the Foreign and Commonwealth Office shouldn't be viewed simply as a naughty step for troublesome frenemies?