Kate Middleton has the cutest nickname for son Prince Louis.

During the 5-year-old’s first royal engagement on Monday, the Princess of Wales called her youngest child “Lou Bug” as he roasted marshmallows.

“Pop [those skewers] in the fire, Lou Bug,” she said as they stood around a campfire at 3rd Upton Scouts Hut in Slough.

Middleton, 41, also asked her son to say “thank you” when he was handed a s’mores.

Prince Louis’ mom called him “Lou Bug” during a Monday outing. ZUMAPRESS.com

Kate Middleton used the nickname during his first royal engagement. ZUMAPRESS.com

The mother-son duo enjoyed s’mores. ZUMAPRESS.com

The future queen has also given nicknames to her and Prince William’s other two children: Prince George, 9, and Princess Charlotte, 8.

Not only did Middleton call her daughter “Lottie” during a March 2019 Northern Ireland trip, but William, 40, referred to the little one as “Mignonette” two months later. (The French word means “dainty” or “cute.”)

The previous year, Middleton called Charlotte “Poppet” as they Christmas shopped in a discount store.

The 5-year-old also pushed a wheelbarrow. POOL/AFP via Getty Images

As for George, he is rumored to be called “PG” at school, which his parents have yet to confirm in public.

The Prince of Wales did, however, reveal in 2018 that George calls him “Pops.”

During the coronation concert on Sunday, Charlotte pointed at her dad onstage and called him “Papa.”

His sister Princess Charlotte’s nicknames are “Mignonette,” “Poppet” and “Lottie.” Daniel Leal-AFP/POOL supplied by Splash News / SplashNews.com

While Charlotte and her siblings have rarely been caught on camera speaking, the trio did make headlines in 2020 for asking David Attenborough questions about animals.

The little one gave more glimpses of their personality over the weekend, from Louis yawning throughout the coronation to Charlotte and George dancing to Katy Perry and Lionel Richie.

George had a major role in the ceremony as one of King Charles’ Pages of Honor, while Charlotte walked in holding Louis’ hand.