It has been established as a fact that Count Montholon had been saddled with the nickname "Liar" among the British officers in place at St. Helena during the captivity of Napoleon. But it seems that the disparaging nickname has not been used other than for a certain period of time and, possibly, Montholon was not even aware of it. This article attempts to trace the origin and reason for this nickname, a topic that few have examined.
This nickname has only been discovered in recent years, after the publication of Gorrequer's secret diary in 1969: it is this text that has made it known for the first time. And at that time, proponents of the poisoning theory, who were looking for a culprit for the mischief they have done their battle, were able to jump on this detail and have given rise to a detail that had been forgotten for over a century. The secret diary indeed mentions, on 14 July 1818, a letter that Montholon had sent several months earlier to the Governor Sir Hudson Lowe to request permission to employ the cook La Roche at Longwood. In his diary, Gorrequer wrote:
On receiving Bugiardo's letter about the cook,... 
Translation: Bugiardo is the Italian word for Liar
Extract from Gorrequer's secret diary
I will not embark here on a discourse about the poisoning theory but suffice to say that, for many reasons, its proponents wished to demonstrate that Montholon was a dubious character who played a double game. He then became the perfect person to suspect of some odious crime. This occurrence of Gorrequer qualifying him as a liar was understood to be so much revealing of the "crime". But, while they took this surname as a sufficient proof, they, to my knowledge, didn't attempt to examine why it had been used. Or, more specifically, why Gorrequer used it in his diary?
Let's begin by noting that, rightly so, the editor of Gorrequer's secret diary correctly warned the reader by a note that, most of the time in his diary, Gorrequer actually nicknamed Count Montholon as "Veritas" ... In fact, the nickname "Bugiardo" was rarely used in the diary. Obviously those who have preferred to see a "liar" in Montholon have overshadowed this second nickname, "Veritas", although it was the one most in use. Whatever their reasons, tagging Montholon definitely as "Liar" hardly reflects the historical truth.
Count Montholon, the co-called "il Bugiardo"
Then let's stress that Gorrequer used the Italian word Bugiardo which indeed means Liar, rather than having used, say, Menteur in French or Liar in English. This detail is especially important because Gorrequer had used English nicknames for other French captives, such as Shrug for the Grand Marshal Bertrand, who was probably having the twitch of a shoulder shrug, and more importantly Neighbour for the illustrious prisoner Napoleon otherwise known as "General Buonaparte" in the official British reports. So why did Gorrequer used the Italian word Bugiardo while Montholon did not even speak nor understand Italian?
In fact the use of that language for this nickname provides us a clue as for its origin and, better as to who had used it first. Those who spoke Italian or Corsican at Longwood included principally Napoleon and Dr Barry O'Meara. Anyb ody who would peruse the famous Lowe Papers in the British manuscripts would realize that the Irish doctor very often used the Italian language to report the exact words pronounced by Napoleon himself in their conversations, probably to avoid of being accused of having misunderstood or mistranslated the captive's own words. With the book recently published, Inside Longwood, which provides for the first time a complete transcription of the manuscripts of the conversations that took place at Longwood between Napoleon and his Irish physician, the reader will notice that Napoleon had himself accused Montholon of lying for the first time in one conversation (which obviously took place in Italian) he had with O'Meara in March 1816.
Dr Barry O'Meara, nicknamed "Magnesia"
Why was Napoleon annoyed at Montholon? Following a circumstance which took place on the night of 11 February 1816, Montholon had made a complain to Admiral Cockburn through the intermediation of Dr. O'Meara, who was at that time used by both sides as a communication channel. But when Napoleon heard of this complaint, he treated Montholon with every name and prohibited him from lying again. Dr. O'Meara wrote to a friend in London, John Finlaison who worked at the Admiralty, a private letter to describe Napoleon's anger towards his companion, who had been rather unfortunate to cause the imperial wrath:
Napoleon broke out into several invectives against Montholon whom he called a Coglione , Imbecile &c &c and only fit to go into the kitchen, [...]
This has in a great manner opened his eyes to Montholon as he scarcely spoke to him for several days, and on one occasion when Montholon was going into the town for some business, Napoleon said to him: "Now Montholon, do not bring me back any lies as news, as Marshal Bertrand is going to town tomorrow and I will then hear the truth." 
It goes without saying that the doctor, having no regard for the couple Montholon, was also quick to report this circumstance at the mess of the 53rd Regiment, who was camping at Deadwood, and certainly gave a good laugh to his fellow officers, and he certainly mentioned it also at his own table, at Longwood, where he used to dine with Captain Poppleton, the orderly officer, and the Polish officer Piontkowski, one of the captives.
The epithet of liar lasted a long time and when General Gourgaud came to be in conflict with Count Montholon, especially after the departure of Count Las Cases at the end of 1816, he affixed a graffiti on the wall of his own room by writing:
Montholon = Ment au long 
Translation: Lies along.
Of course, just after Gourgaud's departure from Longwood in early 1818, the room was emptied and the walls promptly repaint !
Dr. O'Meara had then adopted the nickname bugiardo for Montholon as shown in a note he sent some time later to Sir Thomas Reade, speaking about the Count, shortly before Madame de Montholon gave birth to a daughter. He had written, in Italian:
Che se non fosse bugiardo e vile, sarebbe galantuomo, ma covandorze questi due difetti e brave uomo. 
Translation: if he was neither a liar nor a coward, he would be gentleman because, if we ignore these flaws, he is a good man.
Then, after the birth, the doctor wrote explicitely to Major Gorrequer:
Montholon, better known here by the appellation of "il bugiardo"... 
By "here", O'Meara meant Longwood, or indeed, in Napoleon's mind.
Gorrequer, who was secretly taking notes as a diary, in where he adopted secret nicknames for each individual of the tragicomedy taking place in St. Helena, he simply found a ready nickname for Count Montholon: Bugiardo ! The Governor was himself saddled with the nickname of Mach or Machiavel, because of the devious mind with which he supervised the custody of his illustrious prisoner. But, soon enough, in his secret diary, Gorrequer adopted a new nickname for Count Montholon: Veritas. Was it done by irony or by acknowledgement of a "repentance" from the Count who then was considered to behave honourably?
In the end, Napoleon would have probably been the reason for the use of nicknames in St. Helena. Because he, himself, was in the habit of giving nicknames to his servants (e.g. the Mameluke Aly), his companions (e.g. Gorgotto for Gourgaud), his horses (the horses of Longwood had been renamed after those he had riden during his military campaigns, such as Sheikh, the horse he owned at the time of his death). Napoleon had even renamed his first wife the Empress Josephine who, instead, had Rose as first name: Napoleon did not like Rose !
Dr. O'Meara has simply adopted this same attitude in his conversations in the island and in his private letters. He specifically mentioned that Count Las Cases was called the Jesuit in Longwood, and so on. So, it is naturally that Bugiardo was the adopted name for Montholon, thanks to Napoleon ! But, over time, Napoleon greatly appreciated the services of his officer, and Montholon became Veritas.
And, who knows if the very idea of Gorrequer to use humorous nicknames in his secret diary had not been borrowed from this very imperial habit ...
 Kemble, James, Gorrequer's Diary, 1969, p. 69, see also note 45 p. 273.
 Benhamou, Albert, Inside Longwood, 2012, p. 39.
 In Italien: a jerk, a word that Napoleon used frequently.
 Benhamou, Albert, Inside Longwood, 2012, p. 39, citing Lowe Papers, ADD 20146.
 Macé, Jacques, Le général Gourgaud, 2006, pages 169-170, citing unpublished papers from Saint-Denis, the so-called Mameluke Aly (private collection).
 Lowe Papers, ADD 20115, note from Dr O'Meara to Sir Thomas Reade, 12 June 1816.
 Lowe Papers, ADD 20115, note from Dr O'Meara to Major Gorrequer, 21 June 1816.