adjective

Difficult to understand; obscure.

an abstruse philosophical inquiry

obscure,arcane,esoteric,little known,recherché,rarefied,recondite,difficult,hard,puzzling,perplexing,enigmatic,inscrutable,cryptic,Delphic,complex,complicated,involved,above one’s head,over one’s head,incomprehensible,unfathomable,impenetrable,mysterious