Definition: related to the underworld, the afterlife, or the occult
From the Greek word “khthon” means “beneath the Earth.”
This is likely the root of “Cthulhu.” From Wikipedia:
Invented by Lovecraft in 1928, the name Cthulhu was probably chosen to echo the word chthonic (Ancient Greek “of the earth”), as apparently suggested by Lovecraft himself at the end of his 1923 tale “The Rats in the Walls”.
The “cht” prefix is unique. I went to a Scrabble word chooser, and it only had two words starting with that prefix – this word, and the derivative “chthonian.” It had nothing for “cthu.”
An English word list from the University of Michigan only had “chthonian.” It also had nothing for “cthu.”
Why I Looked It Up
From the novel REAMDE when two characters are discussing the origins of magic items in on online role-playing game:
There is nearly always a chthonic link.