"Shut Up and Leave Me Alone" is the name given to an otherwise nameless and faceless minor male character in Peanuts. He first appeared, as part of a storyline in which some of the characters went to summer camp, on July 21, 1971, which was also the summer that Marcie was introduced. He is Charlie Brown's tent mate.
When Charlie Brown goes over and introduces himself to his new tent mate he receives the response, "Shut up and leave me alone". In fact, every time Charlie Brown says anything to him, whether telling him it is lunchtime, or telling him there is an astronomy lesson, the tent mate replies, "Shut up and leave me alone", hence his nickname.
When Peppermint Patty and Marcie come to visit Charlie Brown, Peppermint Patty introduces herself to the tent mate. He gives her his usual response of "Shut up and leave me alone". This makes the girl really upset because she assumes Charlie Brown forced him to say that.
At the end of the summer, when Charlie Brown is leaving camp, he says he feels upset to part with his tent mate, and will never forget his parting words. "Shut up and leave me alone".
After camp, in the strip from August 23, 1971, Charlie Brown writes him a letter, and is surprised to receive a letter in reply, but the response is, of course, "Shut up and leave me alone."