abstruse (adj.)

1590s, "remote from comprehension," from Middle French abstrus (16c.) or directly from Latin abstrusus "hidden, concealed, secret," past participle of abstrudere "conceal, hide," literally "to thrust away," from assimilated form of ab "off, away from" (see ab-) + trudere "to thrust, push," from PIE root *treud- "to press, push, squeeze" (see threat). Related: Abstrusely; abstruseness.