Recondite
/ˈrɛkənˌdaɪt/ adjective hardDealing with a subject so obscure or specialized that only a select few can understand it.
The professor's lecture on medieval tax law was so recondite that half the class stopped taking notes.
Memory Trick
Recondite comes from Latin 'reconditus,' meaning hidden away or put back into storage -- recondite knowledge has been tucked so far out of sight that only specialists ever dig it back up.