The quality of being difficult to understand or mysterious; something that is hard to comprehend because it's complex or obscure.
From abstruse (Latin abstrusus, 'hidden away, remote') + -ness suffix forming nouns. The Latin root comes from abstrusum, past participle of abstrudere ('to thrust away'). The meaning evolved from 'physically hidden' to 'mentally difficult to access or understand.'
This word became really popular in 18th-century academic writing when philosophers wanted to describe complicated ideas in an impressive way. Interestingly, using a word like 'abstruseness' to describe something often makes it sound more intellectual, even if 'difficulty' would work just fine!
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