anti-joke
Explanation
The Joke
The comic shows a woman asking a man: "Hey, want to hear a reverse anti-joke?" He asks, "What's that?" She explains: "It's a joke that appears to have no punchline, but then WHAM! The punchline comes out of nowhere right at the end!" The man responds flatly: "No thanks."
The joke is self-referential and operates on multiple meta levels. An "anti-joke" is a joke where the expected punchline is replaced by something deliberately mundane or literal, subverting the audience's expectation of humor. A "reverse anti-joke" would therefore be a joke that seems like it is going nowhere but then delivers a real punchline at the end -- which is, of course, just a regular joke. The woman has invented an elaborate name for something that already exists. The man's flat "No thanks" serves as the actual punchline, functioning as an anti-joke ending to her description of a reverse anti-joke.
The Humor
The humor is deeply meta-comedic. The comic itself becomes a demonstration of the very thing being discussed: the woman's explanation of a "reverse anti-joke" sets up an expectation that she will then tell one, but instead the man simply refuses to participate, which is an anti-joke ending. So the comic about a reverse anti-joke is itself an anti-joke. The audience is left holding the comedic equivalent of a Mobius strip, unsure whether the joke landed, failed on purpose, or was never a joke at all -- which is, of course, the joke.