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.
"Hahahahaha! Trigonometry. That old stuff. You're having a tough time with that," laughed my know-it-all sister, Megan.
"Well students. You actually have to discern the entomological aspects of it." "Well, DUH! No shit, Sherlock. If you can't, that's pretty sad," said a know-it-all in the class.
"I don't get how to fix the TV," said my father. "Omg! You don't know how to fix a TV set. Lmao! Who doesnt know how to fix a TV set, take it apart and put it back together again. That's so obvious."
Abby said "no duh" to nearly everything she heard during the study on insects. She is such a know-it-all.
"All you do to fix it, is turn it off and then tighten some of the wiring. Duh," said Kya. Kyra had never tried fixing a computer in her life. She was just acting like a know-it-all.