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Kilroy was here is an American popular culture expression, often seen in graffiti. Its origins are indistinct, but recognition of it and the distinct doodle of "Kilroy" peeking over a wall is almost ubiquitous in the US.
The same doodle also appears in other cultures, but the character peeping over the wall is not named Kilroy but Foo. Australian children write "Foo was here" under the illustration, a habit possibly inherited from the United Kingdom.