The Queen is affectionately known as GaGa to her grandchildren, but her family also penned another nickname for Camilla before she became Her Majesty.
It comes as a new royal book has lifted the lid on Charles' first year as King and his coronation.
Camilla, who is mother to Tom Parker Bowles and Laura Lopes, was given the nickname Lorraine by her family in a nod to her future royal role.
The play on the French word for Queen, La Reine, amused the then-Duchess of Cornwall, but not the then-Prince of Wales, now King, the Daily Mail reports.
Royal writer Robert Hardman, in his new biography, Charles III, out on Thursday, said a friend of Camilla's told him: "She always saw the funny side of that – even if Prince Charles did not."
Before Charles and Camilla's wedding in 2005, royal aides said Camilla did not want to be Queen and "intended", when the time came, to be known as Princess Consort instead, a title which has never been used before in British history, even though legally she would still have been the Queen.
Back in February 2022, Queen Elizabeth II expressed her wish that her daughter-in-law be known as Queen Consort.
In her Platinum Jubilee message to mark the historic 70th year of her reign, the late Queen said: "And when, in the fullness of time, my son Charles becomes King, I know you will give him and his wife Camilla the same support that you have given me; and it is my sincere wish that, when that time comes, Camilla will be known as Queen Consort as she continues her own loyal service."
Buckingham Palace began using "The Queen" instead of "Queen Consort" on the day of the coronation on 6 May 2023.
Royal biographer Robert Hardman reveals that it took Camilla some time to adjust to her new role and realise that mentions of "Her Majesty" were referring to her.
Robert was also one of the producers of the BBC documentary, Charles III: The Coronation Year, which aired last Boxing Day.
The Princess Royal was among the interviewees in the film and said of her sister-in-law Camilla: "I've known her a long time, off and on. Her understanding of her role and how much difference it makes to the King has been absolutely outstanding. This role is not something that she would have been a natural for, but she does it really well. And she provides that change of speed and tone, she's equally modern."
The King and Queen spent Christmas at the Sandringham estate in Norfolk, but since New Year, they have been staying at their Scottish home, Birkhall.
Charles and Camilla were pictured driving to a church service at Crathie Kirk through a snowy Balmoral last Sunday.
LISTEN: Inside the royal family's Christmas