The book on ancient semiotics was so esoteric that only a few experts could understand it.

esoteric 来自希腊语，由前缀 eso-（内部的）和词根 -teric（与神秘知识相关）组成，原指古希腊神秘宗教中只传授给内部成员的知识。在现代英语中表示"深奥难懂的、只有特定人群才能理解的"，常用来形容专业性极强的学术内容或神秘学说。