Used as an expression when someone does something so randomly stupid its funny. its sort of like, the action is the word after the "u know ur cool when" so "u know when ur cool when (you've peed in a can)"

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.