Part of speech: Adjective
difficult to penetrate; incomprehensible to one of ordinary understanding or knowledge
Example: the professor's lectures were so abstruse that students tended to avoid them; a deep metaphysical theory; some recondite problem in historiography
The word "abstruse" was borrowed from Latin abstrūsus, which was borrowed from French abstrus. Many other languages have similar words:
Legal and medical professions still use Latin phrases because of its precision and historical authority.