Despite claims this phrase originated on "The Simpsons", it actually existed long before it was used in the show. The phrase is intended to portray the distortion of facts when passed from person to person.

Its use in "The Simpsons" was accurate; Bart starts a rumour by whispering to one person who whispers it to another, etc. By the time the rumour reaches the front of the crowd, the phrase "Purple monkey dishwasher" has been added to the end due to people mishearing the original rumour as it passed from person to person.

Due to the popularity of "The Simpsons", the phrase has since become a commonly used response to opinions stated based on hearsay and unconfirmed rumours.

Who fucking knows anymore? First, it meant factual, and now it means fictional. What the fuck, internet... what the fuck?

A nonsensical phrase uttered at the end of a 'Telephone-pass it on' gossip game/parody in an episode of The Simpsons.

*In "The PTA Disbands", an episode of The Simpsons, Bart attempts to spread "Skinner said the teachers will crack any minute" throughout the crowd of Springfield Elementary School teachers. By the time it reaches Mrs. Krabappel, it has turned into "Skinner said the teachers will crack any minute, purplemonkeydishwasher."

*Taken from Wikipedia

A parody of the game 'telephone' used on an episode of The Simpsons. Instead of an original message becoming jumbled by person to person contact, the message is perfectly repeated at the end...but with the addition of the words "purple monkey dishwasher."

A nonsense phrase seen widely in an early Simpson's episode. Added at the end of a Chinese whisper, ironically, as there are few if any phrases which can be mistaken for "purple monkey dishwasher". For this reason, it is sometimes used to check if communications are working properly, instead of ie. "testing" "mic check" "test test 1 2 3 check" and/or "syphilis".

A dishwasher made by the world famous 'Purple Monkey' brand. Comes with one year garantee.

Originating from an episode of the Simpsons (episode 2F19 "The PTA Disbands")
It has taken on a comical nonsensical interjectional use... appropriate as comic relief or as a tension breaker