Madame L loves this Latin word meaning the "whatness" or the essence of a person, though she has never used it in a sentence and probably never will, at least in a spoken sentence.
And the only way Madame L can think of to use it in a written sentence is by expanding on the definition:
According to Wikipedia, the word "quiddity" describes "properties that a particular substance (e.g. a person) shares with others of its kind." It was used by medieval philosophers in contrast with "haecceity," the "thisness" of a person, what makes it unique.
"Quiddity" is also the name of a public radio program and literary journal which encourages poets and other writers from all over the world to share their work.