The 32-year-old singer, real name Charlotte Aitchison, coined the term following the release of her sixth studio album - "Brat".

It references an effortless "it" girl state of mind, reflected in the neon green colour of her album cover.

Explained: What is brat?

Along with the Oscar-winning Eilish, she released a new music video for the Guess remix, directed by Aidan Zamiri.

During the video, the pair can be seen climbing a mountain of lingerie as the American star, 23, drives a bulldozer.

"Thank u for letting me be a part of dis," Eilish said, sharing the music video on Instagram.

Charli XCX teased the Guess remix earlier in the week, posting a photo of her with a guest artist, with their faces cropped out.

But fans were quick to figure out it was the What Was I Made For? singer, as Eilish wore the same rings as she had in recent interviews.

It comes after Charli XCX posted: "Kamala IS brat" after Kamala Harris replaced Joe Biden as the Democratic nominee in the upcoming US election.

The Biden-Harris campaign account changed its banner to Kamala HQ, using the same font and neon green background as the brat album cover.

The term brat has been described by the singer as being representative of someone who might have "a pack of cigs, a Bic lighter and a strappy white top with no bra".

She explained on social media how it is similar to the "girl who is a little messy and likes to party and maybe says some dumb things sometimes".

Charli XCX added this is the sort of person who "feels like herself, but maybe also has a breakdown. But kind of like parties through it, it is very honest, very blunt. A little bit volatile. Like, does dumb things.

"But it's brat. You're brat. That's brat."

Charli XCX was nominated for the Mercury Prize album of the year following her latest release.