taking place secretly or without public knowledge
秘密地；暗中地
a subject or person about which one knows nothing
完全理解不了的科目；完全理解不了的人
accounting has always been &B{a closed book to} me.会计学对我来说一直是一门完全理解不了的科目。
accounting has always been &B{a closed book to} me.
会计学对我来说一直是一门完全理解不了的科目。