Background
Abstruse is an adjective, but can be used as an adverb (abstrusely) or a noun (abstruseness). Meaning difficult to comprehend, abstruse was first used in Europe in the mid-1500’s.

Derived from the French word abstrus, or its source, the Latin word abstrūsus (hidden, concealed), abstruse can be broken down into abs (away) + the Latin verb trudere (to push). In addition to the adjective, adverb, and noun, there was a verb form abstrudere, but this part of speech became obsolete by the 17th century. 

Abstruse has been commonly confused with the word obtuse (dull, blunt). Obtuse was originally used to describe angles (obtuse angles) or a person who is slow of mind, but because of the confusion with the word abstruse, obtuse now can be used in the phrase obtuse languages in which the definition changes to mean hard to comprehend. 

Used in a sentence 
Natahsa is proficient in the field of physics, and is able to make every subject she teaches interesting, even the most abstruse topics.

Synonyms
Arcane, unfathomable, deep 

Main source
https://www.merriam-webster.com/dictionary/abstruse#note-2

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