Not know beans about something
Definition 1 of 1
Pro Tip 1/3American Slang
A casual, slang phrase in American English, often used in informal conversations. 
My cousin doesn't know beans about fixing cars.
Pro Tip 2/3Emphasis on Ignorance
Used to stress someone's complete lack of knowledge on a subject, more than just a casual 'doesn’t know'. 
You're asking the wrong person; she doesn't know beans about politics.
Pro Tip 3/3Avoid in Formality
Not appropriate for formal or professional communications, like academic papers or business meetings. 
In a job interview, avoid saying 'I don't know beans about this'.