Charlie Brown is a little boy from "Snoopy" He is like a loser without any talents, but very friendly and likeable to most the people. Calling someone that is basically referring that person to Charlie Brown. Saying "Fuck you, Charlie Brown" is like saying "Fuck you, talentless stupid loser" Why? cause Charlie Brown is a talentless stupid loser cartoon boy from "peanuts and snoopy" so calling someone Charlie Brown is basically saying they are like Charlie Brown.

I highly suggest you read the Peanuts comic strip. http://www.gocomics.com/peanuts/ This is a classic, perhaps THE classic comic strip in American history. Charlie Brown is the main character. He is not what I would call a loser, but just unfortunate. His dog, Snoopy, is also one of the world's most famous comic book characters. I also suggest the video: Charlie Brown's Christmas, and another famous one: It's the Great Pumpkin, Charlie Brown.

Charlie Brown was a character in the comic strip called "Peanuts". The little boy is likeable, but average and without any noteworthy talent, a loser. So calling someone "Charlie Brown" is a condescending form of address. For example, I know your name, but something negative happens or I notice a problem. I might ask, "What's this, Charlie Brown?"

Charlie Brown is one of the most famous cartoon characters of all time. http://en.wikipedia.org/wiki/Charlie_Brown