I'm sure you've seen it somewhere. It is the line drawing of a little, bald man with a long nose, peering over the edge of a wall. This image, along with the tagline "Kilroy Was Here," dates its history to World War II. During the war, this image was scrawled by soldiers on walls wherever they may have passed. It became almost a contest to see who could put Kilroy in the most unusual place. Although the image lost its popularity after the Korean War, it still pops up in unusual places. Live Science
has a fascinating article that traces the origin of the Kilroy-Was-Here graphic.