A form of graffiti. It involves writing in the tiny space of grout in between tiles in public toilets. The phrases always are made up of some pun using the word grout. Other examples include movie titles, like "The grout, the bad, and the ugly" or simple words, like "groutrageous."
This type of graffiti has no deep meaning, but it is a great example of intellectual fun. It makes sense that it exists primarily around Universities where people are intelligent, yet very young and therefore still trying to amuse themselves and others by doing things like writing on walls. It all starts when one person makes one of these grout puns and it is seen by another student who, while urinating, who then thinks of another pun and writes it on another section of grout. Soon the "groutfiti" spreads and people spontaneously think of puns while pissing.
"Thrre stikes and you're grout"
"The Grout Gatsby"
"The Grout Barrier Reef"
"The Groutain of Youth"