The problem with asking for definitions, of course, is that the evidence just isn't there in sufficient detail to say whether particular cases fit one definition or another.
I like the distinction between brittle and stale, though I don't actually know whether there are enough examples to test whether either term is more appropriate to our period.
Wellington's army was certainly jaded and fed up by the last months of 1812 - tired and disappointed, but I don't think any of the symptoms point to 'combat fatigue'. On the contrary, the evidence still suggests that the wet, cold, hungry, grumbling, miserable soldiers were delighted if there appeared to be a possibility of doing some fighting, rather than continuing to retreat. Maybe that was a characteristic more pronounced among British troops of our period than other armies; but where present, I don't think you can say that the troops were suffering from combat fatigue.
That's about as close as I can get to a definition, I'm afraid; and it has brought me full circle to the position I argued in Tactics, that there isn't much evidence of combat fatigue in our period (as applied to units, not individuals - let alone generals). Still, it is interesting that the concept was familiar, not just to Alexander, who wasn't a veteran, but to Napier and Sherer, who were.
Merry Christmas everyone - I hope no one is suffering from 'Christmas fatigue' (or not yet!)