Paris Hilton Confesses She Stole Her Signature 'That's Hot' Catchphrase from Sister Nicky

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Although Paris Hilton has moved onto "sliving," her name will always be synonymous with "That's hot."

But in her new book Paris: The Memoir, the 42-year-old heiress reveals she actually first heard the phrase from her younger sister Nicky Hilton in middle school.

"At some point, I heard Nicky say 'That's hot,' and it resonated with me," the "Stars Are Blind" singer writes. "I wrote it in my diary and doodled flowers and fireworks around it. It's such a great statement, isn't it? Positive. Unpretentious. The word hot is evocative; there's energy in it."

Hilton saw "That's hot" as "a little positive affirmation."

"Suddenly there seemed to be a lot of things in my world that deserved this little accolade, and I recorded them faithfully in my diary," she writes. "Mom got me markers with glitter in them. That's hot. We learned how to diagram sentences. That's hot. Nicole [Richie] is sleeping over the whole weekend. That's hot."

RELATED: Paris Hilton Celebrates 42nd Birthday with Star-Studded Party Featuring Rebel Wilson and Sister Nicky

Paris The Memoir by Paris Hilton
Paris The Memoir by Paris Hilton

Dey Street Books

Before Hilton took the phrase global, it caught on at her school.

"Pretty soon all the kids in my class were saying, 'That's hot,'" the mom of newborn son Phoenix writes. "Like I made 'fetch' happen!"

RELATED: Paris Hilton Reflects on the 15-Year Anniversary of The Simple Life: 'Timeless and Iconic'

After season 2 of The Simple Life, Hilton decided to trademark "That's hot."

"I wasn't sure what I wanted to do with it; I only knew I didn't want anyone else to get there first," the businesswoman writes.

RELATED VIDEO: Paris Hilton Reveals Mom Kathy Hilton Didn't Know About Son Phoenix Until He Was 'Over a Week Old'

In recent years, Hilton transitioned to using "sliving" in place of "That's hot."

"Sliving is a word I invented a few years ago at a Halloween party. I started to say 'slaying' but took a sharp left toward 'living your best life,' and 'sliving' came out," she explains in Paris. "We all died laughing, but I was thinking, That's a great word. Ima trademark that s--- like yesterday."

The House of Wax actress admits she might have been "slightly tipsy" when she made that decision.

Still, she writes, "It is a great word! It's a movement and a lifestyle."

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Paris: The Memoir is out now.