Meaning
What does That’s the Joke mean?
That’s the joke is a popular online catchphrase, originating from The Simpsons.
It is used to highlight the punchline of an otherwise dry-witted, humorless joke in a condescending way.
Origin
What's the origin of That’s the Joke?
The catchphrase first appeared in Episode 18 of Season 6 of The Simpsons, which aired under the title A Star is Burns on March 5th, 1995.
In one of the scenes, a clip is shown from the stand-up comedy performance of Rainier Wolfcastle (dubbed by Harry Shearer).
When he skips the punchline after a joke’s introduction, he says the line “that’s the joke” then proceeds to kill anyone who mocks his skills as a comedian.
Spread & Usage
How did That’s the Joke spread?
In 2006, a YTMND site was launched, with the catchphrase playing on a loop.
On 4chan, the line started appearing as a filename, such as “that’sthejoke.jpg” on boards like /v/ and /a/ starting in 2008.
By 2009, “that’s the joke” would appear on Tumblr as well as Twitter.
A separated clip of the original Simpsons scene was uploaded to YouTube by DovidDovidson on April 16th, 2011.
The first Urban Dictionary definition on the subject was added on February 7th, 2013.

External resources
- Know Your Meme – That’s The Joke