assiduity – n. diligence, close attention to what one is doing
trenchancy – n. keen perceptiveness
Classical Usage: James throws these two words into one big sentence in Chapter XX about the ex-patriots that Mrs. Touchett spends time with in Paris. Isabel saw them arrive with a good deal of assiduity at her aunt’s hotel, and pronounced on them with a trenchancy doubtless to be accounted for by the temporary exaltation of her sense of human duty.
Classically Mad Usage: Huh? I have no idea what our dear friend Henry is trying to say about these Europe loving Americans. Maybe if I do that old vocabulary trick that I use on my son and substitute a bunch of words I can figure out what he’s talking about. Let’s try it: Isabel saw the Americans arrive very diligently at the caravansary where her aunt stayed, and declared on them with a keen perceptiveness that was because of the short-lived importance of her understanding of respecting people.
That made it worse.