know-it-all

1. One who's always acting as if things that are abstruse or not necessarily easy to understand are pointedly obvious, and often shows a snobbish hostility towards others if they're not showing complete understanding of it: someone who treats you like an idiot for not knowing difficult things or acts like difficults things are obvious is a know-it-all 2. One who expresses their unproven opinions and ideas as if they're obvious known facts as if others should have known a long time ago.