I have managed to use the word
callipygian in a sentence in Crimson Mask, and this pleases me greatly. Not only because its a great word, but because the sheer challenge of finding a way to use it, especially in a fantasy novel.
Writers, more than anyone else, are the keeper words and it is their duty to teach them to the next generation. A person doesn't remember those obscure 20 words they learned every week in high school, but they do remember a word used in a great line from a they book love. Writers SHOULD use challenging words that can be understood by context, because otherwise we're going to lose all the crazy synonyms that make English such a beautiful language to read. That being said, I am 100% opposed to going to the thesaurus to look up obscure words just to use them. If I'm going to flout that I've got 15 different words to describe a blow from a sword, I better damn well have earned those 15 different ways by knowing them beforehand. (To use an example I'm sure my beta-readers appreciate.)
Incidentally, the related searches on the page reveal to me Venus Kallipygos. A famous type of statue of Aphrodite, and one, if Wiki is to be believed (and as they mostly cite other Wiki pages and I don't have time to go through the rest of their sources, I'm certainly not going to vouch for them), that had its own cult.