know-nothing

adjective1

1. •
   Ignorant; characterized by foolish denial of facts or beliefs in that which is not true.

   • All over the world, know-nothing xenophobes are claiming – without evidence – that climate science is rigged. — 2017

noun1

1. •
   An ignorant person; one who lacks knowledge.

noun1

1. •
   (US, historical) A member of a secret antiforeigner political organization in the nineteenth-century United States.