ab ‧ struse /əbˈstruːs, æb-/ BrE AmE adjective formal
[ Date: 1500-1600 ; Language: Latin ; Origin: past participle of abstrudere 'to hide' , from abs- 'away' + trudere 'to push' ]
unnecessarily complicated and difficult to understand:
Maths is a mix of abstruse theory and detailed calculations.