Cuthulu is offspring of Nub, who is offspring of Yog Sothoth and has other relatives.
There must be an all knowing Grinch parent and grandparent that is absent and uncaring in his life.
Who is the offspring of the Nameless Mist. Who is the offspring of Azathoth, the blind idiot. If Azathoth wakes up we’re all fucking goners.
It’s a hierarchy of horrors starting with Boo from Mario Bros, on through The Grinch and Cthulhu, and topped off by Azathoth.
It’s his name. You can see in the school flash back, when he’s still part of civilised society, that his nameplate says Grinch.
But interestingly the book of Who says the term “Grinchy” should apply when Christmas spirit is in short supply.
And we’re given the impression the book of who is an old foundational text. So it would appear the old ladies named him after the term Grinchy. So they’re really just assholes.