What does abstruse mean?
The definition of abstruse
Ab*struse”, a.
Etym: [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.