The Canadian singer (born Claire Elise Boucher) has kept information about their daughter Exa Dark Sideræl Musk relatively private since the baby was born via surrogate in December.
At the time, she revealed in a Vanity Fair profile that Exa Dark Sideræl was also called "Y" for short.
Grimes explained the single letter moniker in a tweet on Thursday. "She's Y now, or 'Why?' or just '?' (But the government won't recognize that)," she wrote.
She's Y now, or "Why?" or just "?" (But the government won't recognize that). curiosity, the eternal question, .. and such.
— 𝔊𝔯𝔦𝔪𝔢𝔰 (@Grimezsz) March 23, 2023
The child's name is meant to represent "curiosity, the eternal question, .. and such," she added.
Grimes and the Tesla CEO are also parents to 2-year-old X Æ A-XII, nicknamed "X."
When Grimes initially shared the couple's daughter's name in December, she revealed that she and Musk also call their daughter Sailor Mars, a nod to the "Sailor Moon" manga series. Exa Dark Sideræl was a compromise for the parents.
“I was fighting for Odysseus Musk,” she told Vanity Fair. “A girl named Odysseus is my dream.”
'X Æ A-12 Musk': Grimes, Elon Musk explain the backstory of new baby boy's name
What does 'Exa Dark Sideræl Musk' mean?
"Exa is a reference to the supercomputing term exaFLOPS (the ability to perform one quintillion floating-point operations per second)," Vanity Fair reporter Devin Gordon explained. Dark is “the unknown. People fear it but truly it’s the absence of photons. Dark matter is the beautiful mystery of our universe,” Grimes told the outlet.
She later texted Gordon a voice memo with the pronunciation of Sideræl – “sigh-deer-ee-el” – both a nod to her favorite "Lord of the Rings" character, Galadriel, and described it as “a more elven” spelling of sidereal, "the true time of the universe, star time, deep space time, not our relative earth time."
Grimes says her toddler with Elon Musk doesn't call her mom: 'I don't identify with that word'
Contributing: Hannah Yasharoff
This article originally appeared on USA TODAY: Grimes, Elon Musk's baby girl is named Y. Here's why, the singer says