One who fails to realize the reality of a situation, and instead continues to support/ballyhoo a person, program, etc.
The sunshine pumper sees that their team and coaches are playing hard equating to good, when in fact the players are not performing adequately and the coaches have failed to properly recruit/train/coach the team.
A cousin pumper is a person who fucks their cousin. Typically these people originate from southern United States.
Bill: "dude did Brandon tell you about that chick Tia he banged last week?"
John: "Tia is his cousin, man..."
Bill: "gross that guy is a cousin pumper?!"
The term ladies give to their man friends who do nothing but pump away. These types of men usually can't be bothered to engage in a bit of friendly foreplay. They are often related to the "pump 'n' dump" breed of man.