A man or boy who thinks he is so smart but is really a big nerd who knows nearly nothing. He is usually made fun of because he loves space movies, war, chess, and is usually a republican. You can point them out of a crowd because they will be the ones in line to get a "Mr. Know-it-all" shirt.
"Hey did you see that guy in the Mr. Know-it-all shirt?"
"No, where is he?"
"He is right there."
"Oh my gosh! He is such a nerd!"
"Yeah, what a giglet! Hah hah hah!"