Musk, 48, confirmed the birth of the baby and shared a photo to Twitter on Monday, revealing his son's moniker when asked by a fan.
On Tuesday, Grimes, 32, took to Twitter to explain the baby's unique name, breaking it down piece by piece.
"X, the unknown variable," the singer wrote first, adding an emoji of two crossing swords. "Æ, my elven spelling of Ai (love &/or Artificial intelligence)."
"A-12 = precursor to SR-17 (our favorite aircraft). No weapons, no defenses, just speed. Great in battle, but non-violent," she continued, adding a white heart emoji. "(A=Archangel, my favorite song) (metal rat)."
For emphasis, she added another sword emoji alongside a mouse.
In a snap shared to Musk's Twitter Tuesday morning, the couple's son appears to be sleeping while an Instagram filter placed tattoos around his eyes and forehead.
The baby is Grimes' first child. Musk shares five sons from a previous marriage. Musk's first son reportedly died of sudden infant death syndrome at 10 weeks old back in 2002.
Grimes, whose real name is Claire Boucher, discussed her pregnancy in March with Rolling Stone. The musician told the magazine she was expecting her child to be nocturnal, as the baby moved "only" at night during her pregnancy.
Grimes and Musk have been dating since 2018.
Fox News' Melissa Roberto contributed to this report