Abstruse (&unr_;), 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.]
[1913 Webster]

The eternal eye whose sight discerns
Abstrusest thoughts. Milton.
[1913 Webster]

2. Remote from apprehension; difficult to be comprehended or understood; recondite; as, abstruse learning.
[1913 Webster]

Profound and abstruse topics. Milman.
[1913 Webster]