If you read it on the internet, it passes the first test of copyrightability (having a fixed form). Then the question is whether ownership can be established. Since there is a whole profession of people who write jokes, there is a good chance that ownership can be established and that the copyright will be registered. That would certainly be the case for the joke whose punchline is "you-twit-face". There might be escapees, like the one whose punchline is "I come to caesar Perry, not to braise him", which the author didn't register, but apparently that one is not out there on the internet, and then if someone were to infringe copyright, the author would have to establish that he actually wrote that joke.
A fair use defense is imaginable, but probably would not be successful. Although the extent of copying is minimal, the effect on market would be huge (the punchline is the whole cash value of the joke). Now, the thing is, if this particular punch line is someone else's IP, you may have violated the TOS by asking the question:
Subscriber represents, warrants and agrees that it will not contribute
any Subscriber Content that (a) infringes, violates or otherwise
interferes with any copyright or trademark of another party
and otherwise if you wrote that lovely punchline, you've irrevocably given away the punchline. And that, ladies and germs, is why you should always hire a lawyer.