not know the first thing about (someone or something)
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. What's an attachment again? Geoff wouldn't know the first thing about how to take care of a baby. Thank goodness Frank is there to help babysit. Why do you presume to know what I would or wouldn't do? You don't know the first thing about me!
not know the first thing about someone or something
Fig. not to know anything about someone or something. I don't know the first thing about flying an airplane. She doesn't know the first thing about John.
not know the first thing aˈbout somebody/something
know nothing at all about somebody/something: I don’t know the first thing about Chinese history.