The phrase you state at the end of an argument or statement to create an illusion of credibility when in reality you don't actually know if what you stated is true but still have personal trust in your knowledge. By stating "look it up" you are actually hoping the person won't look it up and just believe you on the spot.
"Vaccinations are evil. Look it up"
"Humans all deserve to die, we are filthy viruses, look it up"
"I am always right, look it up"
A completely bitchy thing to say when asked a simple question. A great way to set a horrible example for children by acting immature and adding fuel to the fire.
TV Interviewer: Madonna, what do you think about Lady GaGa's song 'Born This Way' reportedly sounding a tiny bit like 'Express Yourself'?