"Persona non grata" (PNG), is the most serious form of censure a government can take against a person with diplomatic immunity and often used by governments as symbolic expressions of displeasure.
The severity of a PNG declaration made it surprising that, on November 4, a press release from the UN Mission to South Sudan (UNMISS) informed the media that one of its staff had been "ordered to leave the country".
Journalists were told little else except that this person had worked in the human rights department and that the order was a "breach of the legal obligations of the Republic of South Sudan under the charter of the UN".
It took a meeting with the Ministry of Foreign Affairs to confirm that this was a formal PNG, an important fact omitted from the UN statement and something which UN spokespeople refused to confirm on the phone.
Significantly, the statement (issued by UNMISS) didn't say who the person was, why they had been ordered to leave, what the terms of the order were or whether the UN stood by this person's work.
It is common knowledge that the individual declared PNG is a British citizen called Sandra Beidas.
An email sent to her UN address resulted in a prompt phone call from the communications department of UNMISS informing me that if I tried to contact any UN staff directly again "it would no longer be possible for the UN Mission in South Sudan to assist me".
Why such delicate language?
According to a government spokesman, the sensitive choice of wording was a request made by the UN.
Apparently UNMISS didn't want it to appear as though it was being attacked as an institution – rather it wanted to make absolutely certain that the focus of the government’s displeasure was an individual, Beidas.
What the officer is accused of isn't being made public, the UN wouldn't make any comment at all, and the government would only say that she was considered to be "biased against the government".
However, there are rumours that Beidas had worked on a report about human rights violations in Jonglei state.
Both Amnesty International and Human Rights Watch released reports earlier this year accusing the army of abusing civilians of the Murle tribe in the course of disarming them, an accusation later echoed by the UN, but in more muted tones.
That the government took exception to an individual over these reports is perhaps the strangest aspect of a strange story.
Such publications are produced by a department and therefore don't carry the names of individual authors.
Presumably such a measure can be useful when it comes to the sensitive nature of work in areas like human rights, as it ought to insulate members of staff from being singled out.
There hasn't been any comment from the UN about whether or not it stands by the work of this officer.
All we know is that the individual has been sent to Uganda, "pending a decision on her future status".
I asked the government spokesman how it knew what Beidas had been working on, given that the authors of reports are anonymous.
He replied that when it comes to human rights officers, the government "knows where these people go, who they talk to, who they meet and what they do".
The declaration of Beidas as PNG might not seem to be a very significant event: she wasn't particularly senior, not even the head of the human rights department.
But the consequences could be far reaching, and the incident has caused alarm among South Sudan's human rights community.
Journalists, NGOs and human rights activists are wondering how they can carry out their work with confidence when even an institution as influential as the UN isn't able to protect its staff.