behind closed doors

taking place secretly or without public knowledge

秘密地；暗中地

closed book

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.

會(huì)計(jì)學(xué)對(duì)我來(lái)說(shuō)一直是一門(mén)完全理解不了的科目。