In December 1946 the New York Times credited James J. Kilroy, a welding inspector at the Bethlehem Steel shipyard in Quincy, Massachusetts, with starting the craze. Usually, inspectors used a small chalk mark, but welders were erasing those to get double-paid for their work. To prevent this, Mr Kilroy marked his welding work with the long crayoned phrase ("Kilroy was here") on the items he inspected. The graffito became a common sight around the shipyard and was imitated by workers when they were drafted and sent around the world. As the war progressed, people began opening void spaces on ships for repair, and the mysterious Mr Kilroy's name would be found there, in sealed compartments "where no one had been before."

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.

... was here.

When your cock head peeks up over the top of your jeans or underwear elastic band

me, my fuckin name is kilroy, so fuck you

1 : Rocky Martin... DUHRHHHH...
2 : And me

A human like sub species, who prefers dark enclosed spaces. Usually has two feeble arms which it is unable to support fully, and often collapses in a fit of panic at the call of a siren. In popular slang, a kilroy is someone who shows no sense of self respect or makes no effort with their physical appearance.