Dear Wonderful Weird Readers,
Thanks especially to Weird Wynn, AKA Jeff, for his comment on quiddity.
Madame L also wondered if the brilliant J.K. Rowling took the name of her wizards' game, quidditch, from that Latin-derived word, quiddity, which means the essence of a thing.
But it's not so, according to the author herself, in a 1999 radio interview with Diane Rehm:
JKR: I love making up words. There are a few key words in the books that wizards know and muggles, as in us - no-magic-people, don't know. Well, "muggle" is an obvious example. Then there's "quidditch." Quidditch is the wizarding sport. A journalist in Britain asked me... She said to me, "now, you obviously got the word "quidditch" from "quiddity," meaning the essence of a thing, it's proper nature," and I was really really tempted to say, "yes, you're quite right," because it sounded so intellectual, but I had to tell her the truth, which was that I wanted a word that began with "Q" -- on a total whim -- and I filled about, I don't know, 5 pages of a notebook with different "Q"-words until I hit "quidditch" and I knew that was the perfect one - when I finally hit "quidditch."
If you're a fan of the Harry Potter books, you may enjoy reading that entire interview.