know-it-all

n. One who knows everything; hence, a person who makes pretension to great knowledge, especially one whose didactic conversational habit conspicuously reveals his belief that he has superior knowledge on many subjects; a wiseacre; a know-all; -- usually ironical. [ Colloq. & pejorative ] the use of this term implies that the speaker disapproves of this behavior, and may think that it is unjustified [ 1913 Webster +PJC ]