A closed book

日常英语口语 Spoken English by 每日美语

a closed book完全不了解，搞不懂 closed adj. 关闭的；合上的 与an open book意思相反 He issuch a closed book, he never talks about himself.他这人很封闭，从来不谈论他自己。 I don’t know much about Economics,it is a closed book to me. 我不太懂经济学，对我来说就像天书一样。