Quoting from "The Torturers Tale", WD241:
"He saw creatures sprawling across the realm of our people, ugly mon-kei like you humans and the brutal O, the insufferable Kroots and others."
Now, as far as i can tell, this makes "mon-kei" (which should be mon-keigh, if I am not wrong) point at all the mentioned races, that is humans, orks and kroots + others. Normally, mon-keigh is used only about humans, but I begin to wonder if this word is used to define all undeserving races of the universe.