数典忘祖 Shudian wangzu
Explanation
数典忘祖，比喻忘本，也比喻对本国历史的无知。典指历来的制度、事迹。
Shudian wangzu, literally meaning "forgetting the history and the ancestors", is a metaphor for ingratitude and forgetting one's roots.
Origin Story
晋国典籍官籍谈奉命去周朝参加周厉王的葬礼。周景王设宴款待他，询问晋国为何没有进贡礼物。籍谈回答说，晋国受封时没有得到周朝的赏赐，所以无需进贡。周景王严厉斥责籍谈数典忘祖，不敬先王。籍谈辩解说他只是陈述事实，并没有忘记祖先，但周景王根本不听，认为籍谈的行为是对先祖的不敬，是对历史的无知。
A Jin official, Ji Tan, was sent to Zhou to attend the funeral of King Li. King Jing of Zhou asked why Jin hadn't paid tribute. Ji Tan replied that Jin hadn't received any when it was founded, so it didn't need to. King Jing sharply rebuked him for his disrespect to his ancestors and history. Ji Tan defended himself by saying he was only stating facts, but King Jing refused to listen, condemning his behavior as disrespectful and ignorant.
Usage
常用来批评那些忘本的人，忘记自己祖先的功德，忘记民族的传统，或者对本国历史无知的人。
It is often used to criticize those who forget their roots, the achievements of their ancestors, the traditions of their nation, or who are ignorant of their own history.
Examples
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他数典忘祖，竟然忘记了祖先的功德。
ta shudianwangzu, jingran wangjile zuxian de gongde.
He forgot the achievements of his ancestors.
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不要数典忘祖，要铭记我们民族的优秀文化传统。
buya shudianwangzu, yao mingji women minzu de youxiu wenhua chuantong.
Don't forget your roots, our ancestors and their achievements.