Send us Fan Mail Don't be that guy" is a common phrase used to caution someone against acting obnoxious, inconsiderate, or socially inappropriate. It means you should avoid being the person who ruins the mood, causes a major inconvenience for everyone else, or acts in a way that is universally frowned upon.Depending on the context, "that guy" usually refers to: The Know-It-All: The person who interrupts a conversation to correct a minor mistake or pedantically argue a trivial point. The Party...