The line comes from Looney Tunes, the Warner Bros. cartoon series, where Porky Pig delivered it at the end of many shorts as the series closed. Porky, who first appeared in 1935, became closely linked to the phrase because his stuttering delivery made the sign-off itself a joke. As the cartoons were rerun on television and later shared online, the ending became shorthand for a finished performance. Creators still use it today because it is instantly understood, even by people who have never watched a full cartoon, as a playful way to say the show is over.