Definition of not know from nothing in US English:

not know from nothing

phrase

North American
informal
  • Be totally ignorant, either generally or concerning a particular thing.

    ‘she shakes her head while you talk, as if to say you don't know from nothing’
    • ‘He knew from nothing about playing a piano, but his daughter did and from what she'd told him Steinway was one of the premier piano makers in the world.’
    • ‘You don't know from nothing about girls, do you?’