closed book

• Une personne, un sujet ou une chose difficile à comprendre, à connaître ou à interpréter ; quelque chose de totalement inconnu ou mystérieux pour quelqu'un.

A subject, person, or topic that is difficult to understand, know about, or interpret; something that is completely unknown or mysterious to someone.

1. Mystery or unknownB2informal

   A subject or person that is difficult to understand or know about.

   Livre fermé

   • His past is a closed book to everyone.
   • Quantum physics is a closed book to me.
   • She remains a closed book, even to her closest friends.

2. Finished matterB2informal

   A topic or issue that is no longer being considered or discussed.

   affaire classée

   • That argument is a closed book; let's move on.
   • The investigation is now a closed book.
   • For the team, last season's failure is a closed book.