old-fashioned adverb conjunction
of what or which 关于什么；关于那个
I know whereof I speak (= I know a lot about what I am talking about).
我知道自己在说些什么。
They killed thousands of the inhabitants, whereof many were innocent children.
他们杀害了数以千计的居民，其中许多是无辜的孩子。
He sees hostility, the cause whereof he does not know.
他看到了敌意，却不知道其原因。