Screenwriter Peter Morgan reportedly said: "I inquired in royal circles and was told on very good authority that that is what the duke sometimes calls the Queen."
Back in 2018, The Sun also rounded up what each royal family member is called by their loved ones, and some were quite surprising.
Her Majesty's great-grandchildren Prince George and Princess Charlotte like to refer to the Queen as "Gan-Gan," while William jokingly called her "Gary" when he was young after struggling with the pronunciation with "Granny".
Royal writer Richard Kay wrote in the Daily Mail that "The Queen was on hand after William fell over at Buckingham Palace, bawling 'Gary, Gary'.
"A guest who went to help asked who Gary was, assuming it must be a member of the royal household. ‘I’m Gary,’ explained the Queen, as she scooped him up. ‘He hasn’t learned to say Granny yet'."
Prince George and Princess Charlotte also apparently have a sweet nickname for Camilla.
The Duchess of Cornwall told Lady Gaga at the 2016 Royal Variety Performance that her grandkids call her "Gaga".
Prince Harry sweetly calls his father "pa" and is often called "darling boy" in return. This was demonstrated at the prince's wedding, when Prince Charles walked Meghan Markle down the aisle, Harry could be heard thanking his "pa" and Charles sweetly replying.