Kilroy was here is an American popular culture expression, often seen in graffiti. Its origins are debated, but the phrase and the distinctive accompanying doodle—a bald-headed man (possibly with a few hairs) with a prominent nose peeking over a wall with the fingers of each hand clutching the wall—is widely known among U.S. residents who lived during World War II.
The Oxford English Dictionary says simply that Kilroy was “The name of a mythical person.”
One theory identifies James J. Kilroy (1902–1962),an American shipyard inspector, as the man behind the signature. During World War II he worked at the Fore River Shipyard in Quincy, Massachusetts, where he claimed to have used the phrase to mark rivets he had checked. The builders, whose rivets J.J. Kilroy was counting, were paid depending on the number of rivets they put in. A riveter would make a chalk mark at the end of his or her shift to show where they had left off and the next riveter had started. Unscrupulous riveters discovered that, if they started work before the inspector arrived, they could receive extra pay by erasing the previous worker's chalk mark and chalking a mark farther back on the same seam, giving themselves credit for some of the previous riveter's work. J.J. Kilroy stopped this practice by writing "Kilroy was here" at the site of each chalk mark. At the time, ships were being sent out before they had been painted, so when sealed areas were opened for maintenance, soldiers found an unexplained name scrawled. Thousands of servicemen may have potentially seen his slogan on the outgoing ships and Kilroy's apparent omnipresence and inscrutability sparked a legend.
In the postwar United States the mischievous face and the phrase became a national joke. The outrageousness of the graffiti was not so much what it said, but where it turned up. The major Kilroy graffiti fad ended in the 1950s, but today people all over the world still scribble the character and "Kilroy was here" in schools, trains, and other similar public areas.