"Foo was here" is an Australian graffiti

Graffiti

Graffiti is the name for images or lettering scratched, scrawled, painted or marked in any manner on property....

 signature of popular culture

Popular culture

Popular culture is the totality of ideas, perspectives, attitudes, memes, images and other phenomena that are deemed preferred per an informal consensus within the mainstream of a given culture, especially Western culture of the early to mid 20th century and the emerging global mainstream of the...

, especially known for its use during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, but also became popular amongst Australian schoolchildren of post-war generations.

Foo (or Mr Chad) is shown as a bald headed man (sometimes depicted with a few hairs) peering over a wall (usually with the fingers of both hands appearing to clutch over the wall as well), with the simple inscription "Foo was here".

Origins

While its exact origins are unknown, the "Foo was here" graffiti is said to have been widely used by Australians during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 and also during World War II. At some point the Australian phrase combined with a popular British piece of graffiti called Mr Chad, most likely as a result of the two countries' respective armed forces working together in such close proximity.

One source says of Foo that "He was chalked on the side of railway carriages, appeared in probably every camp that the 1st AIF World War I served in and generally made his presence felt". If this is the case, then "Foo was here" predates the American version of World War II, "Kilroy was here

Kilroy was here

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 with a prominent nose peeking over a wall with the fingers of each hand clutching the wall—is widely known among U.S...

", by about 20 years.

It has been claimed that Foo probably came from the acronym for Forward Observation Officer, but this is likely to be a backronym

Backronym

A backronym or bacronym is a phrase constructed purposely, such that an acronym can be formed to a specific desired word. Backronyms may be invented with serious or humorous intent, or may be a type of false or folk etymology....

.

Other sources suggest that Foo was a man that was inspecting the welds in submarines. He had to crawl inside small spaces to do this, his superiors were wondering if he was doing his job so everywhere that Foo went he would write "Foo was here". This has been found on the wreckage of many subs and ships. This trend soon caught on.