recondite \REK-uhn-dyt\ (adjective) - 1 : Difficult to understand; abstruse. 2 : Concerned with obscure subject matter.
"Reginald enjoyed the daily emails as a way to keep his already recondite vocabulary satisfyingly obscure."
Recondite is from Latin reconditus, past participle of recondere, "to store back," i.e., "out of the way," hence "to hide"; itself from re-, "back, again" + condere, "to put away, to store." Thus, recondite knowledge is "hidden" (because of obscurity or difficulty) from the understanding of the average person.