Ed, Edd n Eddy is a Cartoon Network animated series centred on three friends in a suburban cul-de-sac, known for its exaggerated cartoon voices and slapstick scams. The that's not funny line fits that tone perfectly, turning a simple dismissal into a memorable reaction. As short cartoon clips began circulating on soundboards and in video edits, this kind of blunt reaction became a handy shorthand for calling out an unfunny moment. It endures because the wording is short, clear and easy to drop into almost any failed joke.