You don't have the faintest idea what you're talking about.

你根本不晓得你在说什么。

I don’t have the faintest idea what you’re talking about.

【faint，faintly】我一点都不知道你在说什么。

I don’t have the faintest idea what to get Rachel for her birthday.

【idea】我一点也不知道给瑞秋送什么生日礼物。