Abstruse, a.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for abstruse; he lists it under complex, mysterious, obscure.
Where he writes about it:
We speak of an abstruse statement, a complex conception, a confused heap, a heterogeneous mass, a tangled skein, an intricate problem; of composite architecture, an involved sentence; of the complicated or intricate accounts of a great business, the entangled accounts of an incompetent or dishonest bookkeeper. — from complex
In that which is abstruse (L. abs, from, and trudo, push) as if removed from the usual course of thought or out of the way of apprehension or discovery, the thought is remote, hidden; in that which is obscure there may be nothing to hide; it is hard to see to the bottom of the profound, because of its depth, but the most shallow turbidness is obscure. — from obscure
Synonyms
complexmysteriousobscureambiguouscabalisticcloudycomplicatedcompositecompoundconfusedconglomeratedarkdarksomedeepdensedifficultdimdoubtfulduskyenigmaticalentangledheterogeneoushiddenincomprehensibleindistinctinexplicableinscrutableintricateinvolvedmanifoldmingledmixedmuddymultiformmysticmysticaloccultprofoundreconditesecrettangledtranscendentalturbidunfathomableunfathomedunintelligibleunknown