=>To not know anything about someone or something.
I don't know the first thing about computers, so you'll have to explain this to me really slowly.
I don't know the first thing about flying an airplane.
She doesn't know the first thing about John.
I'm afraid I don't know the first thing about car engines.
https://idioms.thefreedictionary.com/not+know+the+first+thing+about
https://dictionary.cambridge.org/zht/%E8%A9%9E%E5%85%B8/%E8%8B%B1%E8%AA%9E-%E6%BC%A2%E8%AA%9E-%E7%B9%81%E9%AB%94/not-know-the-first-thing-about-sth