I think we have a better language for pain than fatigue. Pain is also understood by most to be a disabling symptom, whereas everyone living has been tired, and it hasn't stopped them. So if all it is is fatigue, than it can't be much.
In the past I have described the fatigue -which is my most disabling symptom- as a combination of the fatigue you have when you have the flu, with insomnia. So tired, sick, eyes half lidded and stinging. And at its worse when it leaves you pinned to the bed by thousands of tiny invisible weights. I have thought at these times "better not be a fire, because I would surely burn"
This past year I have suffered with tremendously difficult neurological problems, and has made me rethink the illness. The fatigue now is more painful, it drills in me. It has this numbing quality, and make my brain feel like it's filled with cotton. (Sick 27 years and never had brain fog until about a year ago). Yet these neurologic disorders bring a fatigue that I recognize. I've been feeling it for years, in the severe patches. I think I have had the neurological form all along, though for all these years I strictly thought the fatigue was immune related, because the fatigue really felt to me like the fatigue of mono, of a bad flu.
Sometimes when I just wake up I have a few seconds of consciousness before my body wakes up. That's the time I'm free. Until 2 seconds later, when I start to feel it, the incipient sickness creeps and seeps in, slowly until my body is once again enveloped by it -- a living evolving tortuous sickness, a living part of me that hangs like a mist, but never clears. My mind is free for 2 seconds a day, if I'm lucky, then back to prison.