I think it is simpler than that. I am talking of the generalised unpleasantness that gets called 'fatigue' or 'exhaustion' or 'bleurrgh' and fades into nausea for some, pain for others and 'my arms just won't...' and so on.
Depending on the audience, which I assume it British, from what I read the common British English equivalent would be 'poorly'?
On our side of the Atlantic it's generally closer to feeling unwell, but really there isn't good vocabulary for this as this has to span the "blurgh" of a cold all the way to a bad flu or severe ME. It's a significant issue that I think overlaps with the "hard to describe" problem. This is actually why it's important for patients to work their way through social media and see descriptions and words from other people who at least get close enough.
Not everyone is good with words, and even then it doesn't mean being good at describing this kind of odd subjective experience, and then there's brain fog that muddles things further. Worse still is explaining it to people who not only have their special vocabulary, but can oddly trip on some perfectly fine words for weird historical reasons and/or gaps in training that are similar to people who are blind from birth having no useful vocabulary for colors.
But of course many MDs think this is how people get those ideas in their heads, when really it's just the process of figuring out how to describe things that are difficult to describe, and relying on the wisdom of crowds for the few who pulled it off, even if partly. People do the same thing all the time for other things. Hell, it's the entire basis of "let me teach you about your subjective experience of your illness", but it only works one way in medical culture.
Most of it seems to be down to the rejection of
feeling ill as an accepted term, which is about as useful as rejecting the use of the word red, then asking someone to describe something that is red. I don't feel fatigue or tired. I feel ill. Or sick, but that's too often used for nausea and vomiting, which is weird because it's a generic term.