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Kilroy was here is an American popular culture expression, often seen in graffiti. Its origins are open to speculation, but recognition of it and the distinctive doodle of "Kilroy" peeking over a wall is known almost everywhere among U.S. residents who lived during World War II and through the Korean War.
Similar doodles also appear in other cultures. In an Australian variant, the character peeping over the wall is not named Kilroy but Foo, as in "Foo was here". In the United Kingdom, such graffiti is known as a "chad". In Chile, the graphic is known as a "sapo" (slang for nosy); this might refer to the character's peeping, an activity associated with frogs because of their protruding eyes. In Mexico it is known as "El Fisgon". A very similar figure is "Tosun" in Turkey. Tosun is both a less used male name and also refers to a bullock. It is used as "Bunu yazan Tosun" ("Tosun wrote this"). In Poland Kilroy is replaced with "Tony Halik" (Polish traveller and explorer) or "Józef Tkaczuk", an elementary school janitor, whose name was frequently used in graffitis. Graffiti writings have the form of sentences like "Gdzie jest Józef Tkaczuk?" ("Where is Joseph Tkatchuk?") and "Tu byłem - Józef Tkaczuk" ("I was here - Joseph Thatchuk"). It has appeared in movies as well--at the end of the WW2 war comedy Kelly's Heroes, Kelly and his men leave the doodle in a bank they've just robbed, along with the words, "UP YOURS, BABY". It appears in the 1970 film Patton on the side of a tank.
800px-Kilroy Was Here - Washington DC WWII Memorial

Engraving of Kilroy on the WWII Memorial