Abstruse is an adjective that means difficult to understand or obscure.

Abstrusus (ab STREW soos) is a Latin word for ‘put away hidden.’ From this word, came abstruse. Often used in a literary context, It can be thought of as synonym of words like arcane and esoteric.

Conrad’s books are brilliantly written, but I find them abstruse. Even after three or four readings, I have no idea what they’re about.