不知原委bù zhī yuán wěi

not know the whole story (/all the details/how a thing happened from beginning to end); not understand what has happened
❍ 黛玉虽～，探春等却晓得是议论金陵城中居住的薛家姨母之子——表兄薛蟠，…… (《红楼梦》40) ❶Daiyu did not understand what was going on,but Tanchun and the others knew that they were dis cussing Xue Pan,the son of Aunt Xue in Jinling.
❷Tanchun and the girls told Daiyu,who knew nothing of the matter under discussion,that they were talking about Xue Pan,the son of their Aunt Xue who lived in Nanjing.