A douche who tends to present their douchery to an inactive audience at unorganized sporting events or sports-related video game sessions. One who acts like an asshole during recreational activities to compensate for their inability to do anything well. Also, more simply, that guy who thinks he could fuck your girlfriend because he is better at golf than you.
On the spectrum of relative douchery, a Doucher McGavin falls just between a large rubber dildo used for vaginal irrigation and Kanye West.
Announcer: Now stepping up to the tee, Doucher McGavin!
Person 1: Do we have to listen to his bullshit again?
Person 2: I hope he dies, because I want to be able to come here again without having him sodomize my soul with stories of how great he was in high school.
Kanye West: Yo Doucher, imma let you finish--but back in the day I had one of the best jump shots of all time!