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difficult to understand; unintelligible; abstruse
彼の文章は難解すぎる。
His writing is too hard to follow.
この条文は難解で意味がつかみにくい。
This clause is obscure and hard to make sense of.
その哲学書は難解で何度読んでも理解が難しい。
That philosophy book is so abstruse that it's hard to grasp no matter how many times you read it.