This could potentially be a misspelt word. According to dictionary.com: quiddity is 1. the quality that makes a thing what it is; the essential nature of a thing. 2. a trifling nicety of subtle distinction, as in argument.
Just rescued this from the orphanage, so someone besides myself originally entered it. Can't find much on it anywhere, except for this blog post, which defines it as "the essence, nature, or distinctive peculiarity of a thing."
Fine word, somewhat mysterious. Anyone know any more about it?