Abstruse is a formal word used to describe something that is hard to understand.

// I avoided taking this class in past semesters because the subject matter is so abstruse, but the professor does a good job explaining the concepts as clearly as possible.

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“The EP’s lyrics are suitably abstruse. The title ‘Marry Me Maia’ sounds forthright in its intentions, but the song instead offers cryptic references and obfuscation. The result is like peeping in on a private conversation: fascinating and impassioned but fundamentally obscure.” — Ben Cardew, Pitchfork, 31 Mar. 2025

Look closely at the following Latin verbs, all of which come from the verb trūdere (“to push, thrust”): extrudere, intrudere, obtrudere, protrudere. Remove the last two letters of each of these and you get an English descendant whose meaning involves pushing or thrusting. Another trūdere offspring, abstrūdere, meaning “to conceal,” gave English abstrude, meaning “to thrust away,” but that 17th-century borrowing has fallen out of use. An abstrūdere descendant that has survived is abstruse, an adjective that recalls the meaning of its Latin parent abstrūsus, meaning “concealed.” Like the similar-sounding obtuse, abstruse describes something difficult to understand—that is, something that has a “concealed” meaning.