abstruse

adjective2

1. •
   (formal) Difficult to comprehend or understand; obscure.

   • Be less abstruse, my riddling days are past. — 1671
2. •
   (formal, no longer used) Concealed or hidden; secret.

   • [T]he abstrusest Things / VVhich in the Mindes dark Temper nestling ly, / By you exposed are to every Eye. — 1648