The adjective abstruse means not that known or difficult to understand and comprehend.
Synonyms are perplexing, esoteric, complex, or profound.
The word origins from Middle French abstrus (16c.) or directly from Latin abstrusus meaning “hidden, concealed, secret” past participle of abstrudere meaning “conceal, hide” literally “to thrust away”. The assimilated form of ab meaning “off, away from” + trudere meaning “to thrust, push”.
Our literature professor finds mathematics an abstruse subject.
The case was abstruse even to the prosecutor.
The teacher assigned an abstruse topic to the late comers.