It's an internet fitness industry term, a way to excuse shitty programming.
It goes hand in hand with over diagnosis, usually in the interests of people who then claim to be experts in treating these things (for cash). If you have “CNS fatigue” there might be a chance to get some money out of you for some whacky treatment. But plain old tiredness, well that just needs sleep food and less stress.
I think it was Chuck Norris who invented the term CNS since no one was there to stop him
Not quite. CNS fatigue is an acctual fenomena that is associated with high metabolic stress. pH levels change to a point where signaling from the CNS becomes impaired.
It however, has nothing to do with what some lifters call "CNS fatigue", which is a way they found to describe the sensation of fatigue that is not immediately associated with muscle soreness.
I remember reading an article that suggested Louie Simmons may have had something to do with the popularization of the idea. I also recall reading about some studies that implied CNS fatigue was real, although not very significant (lasting about 30 minutes), wasn't only caused by heavy barbell training, and that the central nervous system follows the SRA cycle along with everything else.
Whenever I heard someone talk about "frying their CNS" I assumed they were just looking for an excuse not to pull heavy.