Abstruse

Ab*struse" (?), a. [L. abstrusus, p. p. of abstrudere to thrust away, conceal; ab, abs + trudere to thrust; cf. F. abstrus. See Threat.] 1. Concealed or hidden out of the way. [Obs.]

The eternal eye whose sight discerns
Abstrusest thoughts.
Milton.

2. Remote from apprehension; difficult to be comprehended or understood; recondite; as, abstruse learning.

Profound and abstruse topics.
Milman.