-noun, from the English perplex and lexicon
1. Collectively, all words with which one is familiar but is unable to define.
2. Words that one would commonly be expected to know based on his or her education or socio-economic class but does not.
3. Particular words, generally of an erudite nature, which one finds perpetually befuddling despite previous attempts to ascertain their meaning.
"No matter how many times I look up fecundity in the dictionary, it always seems to slip back into my perplexicon by the time I next encounter it."
"Four years of college and I all I have to show for it is a massively inflated perplexicon!"