Somebody who thinks they're cool because they know a million definitions on this site. They spend their time bragging on how they the name of every sexual position involving feces and belittle people who don't know every single definition here.
Urban Dictionary Douche: You mean you don't know the difference between a Cleveland Steamer
and an Alabama Hot Pocket
Normal Person: ... I'm gonna go now.
Urban Dictionary Douche: You are so weak sauce!
Normal Person: Get welded.