Johnstown Flood means in slang: Pranks in college that have many names. Most often played in college dorms. Filling up a trashcan about half-way with water. Next, you will place the trash can on top of a door. Run like mad and knock on the door. The water gushes everywhere when the sucker opens the door. It’s the Johnstown Flood because I go to the University of Pittsburgh in Johnstown. The name is fitting, especially if you know the history of the area. (in Slang Dictionary, added by Catherine Sanford)