To seem or affect to know everything about any given topic, and to make an irritating point of demonstrating that knowledge to others. Jim's new boyfriend always acts like he knows it all. I can't stand having a conversation with him about anything!

A person who seems or affects to know everything about any given topic, and who makes an irritating point of demonstrating their knowledge to others. Jim's new boyfriend is such a know-it-all. I can't stand having a conversation with him about anything! There is nothing more annoying than working with a know-it-all who constantly tries to outdo everyone else.