Simple question that I can't seem to easily find an answer to...
In the last book of the series (The Dark Tower), the Crimson King is called 'Los' by, I believe, Dandelo.
However, I cannot seem to remember why he is called this....
I've read the books in another language than the original ones and didn't remember this.
I then checked in my books (in french) and it's also Los'.
As I have the original version in ebook format, I was able to search through the whole seventh book. The first time Los' is used is in Part IV, chapter III: The Castle of the Crimson King.
The first occurrence I can find is when Roland, Susannah and Oy meet Feemalo, Fumalo and Fimalo.
"Since the names we give ourself would be unpronounceable to you [...] you may as well use those Los' used. Him being the one you call the Crimson King."
Then they use the term Los' several time without giving a meaning to it (at least I didn't find it).
"This man is as crazy as Los' the Red" Fumalo said, not without respect.
The only other person using this term is Rando Thoughtful, when caught by Mordred.
"Mordred, son of Los'," he said, trying a smile. "Hile to you, King that will be!"
In this last book, I find the term Los' 10 times. 9 times used by F(ee/u/i)malo and once by Rando.
The Wikipedia page about Dark Tower characters states:
The Crimson King is known by a number of names, including Los' the Red, Ram Aballah, The Aballah, The Kingfish, The Red King, The Lord of Spiders, and The Lord of Discordia.
I don't have yet the possibility to look in the Concordance book but I may soon. I'll check this and come back if there are any details in it.