All couples love to poke fun at each other and it turns out Prince William and Kate Middleton are no exception.

The Sun is reporting that the Duchess of Cambridge, 37, calls her 36-year-old hubby “baldy”.

A source reportedly told The Sun that the nickname came about after Prince William “complained about her endless mane of hair [so ‘baldy’] has been her sharp retort.”

William reportedly forked out $330 on a buzz cut last year at the advice of his wife, following brother Prince Harry, 34, having a go at him about losing his hair.

“Baldy” is a far cry from the slightly more risqué and complimentary nickname Kate had for Prince William while they were dating: “Big Willy”.

But the cheeky name-calling doesn’t stop there.

Prince William has a jokey pet name for his wife in return and is said to call her “DoD”, which is short for “Duchess of Dolittle”.

Although Kate is now a busy mum-of-three and working royal, before she married Prince William, she is said to have been criticised by the Queen for not having a full-time career.

Could the cheeky nickname be a nod to her early days?

One royal source told The Sun: “The royals are not very good at communicating with one another so this is one way around it. Nicknames are a way of taking the family tension out of things.

“They can also be a rather childlike family. They love to play games and they give each other silly presents. I think it’s because they have to be so earnest in their public lives.

“Royals have always played in private. So William and Kate are really carrying on a long held tradition.

“It’s done with affection usually and there are grains of truth about what one feels about another.”

And it’s not just for good humour that the royals love giving each other nicknames, as the secret labels can be useful for security reasons too.

A second Palace insider told the British newspaper: “Nicknames for the royals may seem like a bit of fun.

“But they do sometimes use them to identify one another while not making that information public.”

The pet names of the Duke and Duchess of Cambridge have emerged following their son Prince George, five, confusing a dog-walker by claiming his name is Archie.

Royal aliases continue, with heir to the throne Prince William being called “Willy the Wombat” by his late mum, Princess Diana.

He is said to have been given the sweet moniker after being transfixed by the animal during an official royal tour to Australia when he was two.

And when he was studying at the University of St Andrews, William called himself “Steve” in a bid to blend in with fellow students.

At Kate’s childhood school, St Andrew’s in Berkshire, she was known as “Squeak”, leading to her and her sister Pippa being jointly named “PipSqueak.”

And when she was pregnant, Kate and William referred to eldest son Prince George as “our little grape”.

Now grade one at Thomas’s Battersea, Prince George is known as PG, and his parents now fondly call him PG Tips or just “Tips” after the tea.

Prince George and three-year-old sister Princess Charlotte call their great-grandmother, the Queen, 92, “Gan-Gan”.

And they have also adopted a similar nickname for the Duchess of Cornwall, Camilla, 71, who they call “Ga-Ga”.

Prince Harry hasn’t missed out on the action, being known as “Flash”, which he may have got from his Army days and comparisons between him and the well-loved fictional military Lothario Harry Flashman.

He also goes by the more magical name of “Potter” by friends, in a nod to the fictional wizard, and will also answer to “Gromit”.

And his wife Meghan, 37, also has an unusual collection of names, being called “Flower” by her mum, “Bean” by her dad and is called “Tungsten” by Prince Charles because she’s tough.

The Queen has always been known as “Lilibet” in her family because she couldn’t say her name Elizabeth when she was young.

Prince Philip, 97, calls her “Cabbage” or “Sausage”, and Prince William called her “Gary” when he was little and couldn’t pronounce her name.

Prince Charles and second wife Camilla secretly called themselves “Fred and Gladys” during their long affair while Charles was married to Diana.

Prince Charles is called “Fred” by wife Camilla, who also signs off her cards to him with “Your devoted old bag”, according to the Daily Mail.

Prince Andrew is known as “Air Miles Andy or Randy Andy” by his detractors”, and Prince Edward is called “Steady Eddie”.

Princess Beatrice was given the nickname “Trixie Belle” by her mum Sarah Ferguson, and Princess Eugenie is called “Euge” by the York family.

And Princess Diana was called “Duch” by the Spencer family, and even signed Christmas cards as “Charles and Duch” to the Spencers and Yorks.