shortened from the recurring Saturday Night Live skit, Debbie Downer, a "debbie" is a person who consistently makes negative and cynical comments. She brings everyone down with every comment. On the skit, her conversations killers were always followed by the classic "waaan waaan."
John: Let's go to the carnival!
Jane: I heard more people die on carnival rides than any other cause of death in the US.
John: Thanks for being such a debbie.