What does being that guy mean?

being that guy meaning in Urban Dictionary

usually the one individual you understand that, regardless of how lots of people trust something, whether it is a course of activity or a straightforward reality, ALWAYS disagrees. Ergo title,"Everyone desires to get except THAT GUY". This can be thought to be deliberately or out-of sheer stupidity. After disagreeing anyone will often followup with a statement which completely wrong and false because he "read is someplace" or some one completely unreliable informed him.