I don't know how, what, why, etc. idiom informal
（用于加强批评、惊讶等的语气）真没想到，真不知道
used to add force to criticisms, expressions of surprise, etc.
I don't know how you can eat that revolting stuff! 天知道你怎么吃得下那么恶心的东西！
I don't know why you bother. 真不知道你为何要操心。