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。