In her new book The New Royals, Katie Nicholl claims Charles used to call Meghan "Tungsten", an extremely durable metal that can be put through a lot without getting damaged.
"Charles reportedly nicknamed his future daughter-in-law ‘Tungsten’ because of her toughness and resilience," the royal expert writes.
She claims the nickname came from when the former actress showed resilience during her appearance with Prince Harry's 2018 appearance with Prince William and Kate Middleton at the Royal Foundation Forum.
"It was, according to one aide, the moment William and Kate, who was heavily pregnant, realised they needed to up their game," Nicholl writes in the book. "The Cambridges [now the Prince and Princess of Wales] had already signalled their intention to be more than ‘ornamental royals’ and had, along with Harry, made a huge success of their mental health campaign Heads Together.
"But Meghan was the breakout star of the foursome. She was polished, passionate and funny, using all her TV-honed skills to present her case. That was a wake-up moment for William and Kate when they realised that Meghan was very impressive, very confident and very capable, according to a source."
It's unknown if Charles still uses the nickname for Meghan now, given the tensions between them after she and Harry stepped down as senior members of the royal family and moved to the US.
Meghan's swipe at the royal family
It comes after Meghan seemingly took a swipe at the royal family in her podcast Archetypes this week.
The podcast paused the release of new episodes for the official mourning period following the Queen's death last month.
In the new episode of the podcast, Meghan, along with guests journalist Lisa Ling and comedian Margaret Cho, explored the 'Dragon Lady' stereotype with the former actress telling listeners to disregard the "loud voice coming from a small place".
She also urged listeners to be their "best and true self", saying, "You want to be weird or be sponge-like, be silly or fierce, be curious, or even self doubting or unsure some days and strong and brave on others."
"Whatever it is, that's up to you. Just be yourself no matter what any societal framework or archetype or loud voice coming from a small place tells you that you should be.
It comes after Meghan spoke to The Cut about how she and Prince Harry were "happy" to leave Britain and were "upsetting the dynamic of the hierarchy... just by existing" before stepping down as senior royals.
She added that what she and Harry were asking for wasn't "reinventing the wheel".
Elsewhere in the chat, Meghan reveals that Harry told her how he had "lost" his dad King Charles in the fallout of their move to the US.
Never miss a thing. Sign up to Yahoo Lifestyle’s daily newsletter.
Or if you have a story idea, email us at firstname.lastname@example.org.