closed book       
noun [ S ]  informal uk/ˌkləʊzd ˈbʊk/ us/ˌkloʊzd ˈbʊk/
 一无所知的学科,完全不懂的学科
a subject about which you know or understand nothing
  I'm afraid physics will always be a closed book to me. 恐怕我对物理永远是一窍不通了。 
 Synonyms: enigmamysterysecret