C2 根本不知道
used to emphasize that you do not know something
"Is she going to stay?" "I haven't the faintest idea." “她要留下来吗？我一点儿也不知道。”
I don't have the faintest idea what you're talking about! 我根本不知道你在说什么！
Neither of them had the faintest idea of how to be a parent.
As to what I'm going to do now, I haven't the faintest idea.
I don't think Steve had the faintest idea about what was happening around him.
See also: faint adjective (SLIGHT)