Joke theft is the act of performing and taking credit for comic material written by another person without their consent. This is a form of plagiarism and sometimes can be copyright infringement.
A common epithet for a joke thief is "hack", which is derived from the term, "hackneyed" (Meaning, "over used and thus cheapened, or trite").
From the music hall and vaudeville beginnings of comedy, it was common for a performer to "borrow" from another one. According to Milton Berle, etiquette only required that "the borrower add to the joke and make it his own." At the time there were few chances that a performer from one area would meet one from another and a single twenty-minute set could sustain a comic for a decade. Most jokes at the time were one-liners and there was little in the way of proof of a joke's origin, but the value of each joke was immeasurable to a comedian. Berle and Bob Hope had a long-standing feud due to Hope's accusation that Milton Berle had stolen some of his jokes. Berle never refuted the claim, but instead embraced the title "The Thief of Bad Gag".
Even the most famous of comics have found themselves, knowingly, or unknowingly, stealing material. Bill Cosby admitted to stealing a joke by George Carlin involving an uneducated football player doing a television commercial. Cosby said that what makes the routine his own is the surreal phrase "little tiny hairs." Likewise, Carlos Mencia, many years later performed a bit about athletes and their parents that hearkened back to a Cosby bit from his album, Bill Cosby: Himself.
In the 1970s, joke theft became more prominent with the boom in popularity of comedy. The 1980s and 90s saw the popularity of stand-up comedy continue to increase. With the advent of pay-cable networks, comics were afforded the opportunity to perform their routines unfettered. With this came a new type of joke theft wherein the first comic to tell a stolen joke on some sort of media became the one associated with the joke.
Robin Williams was accused of stealing material from another comic. David Brenner claims that he confronted Williams personally and threatened him with bodily harm if he heard Williams utter another one of his jokes.
For many years, Denis Leary had been friends with fellow comedian Bill Hicks. However, when Hicks heard Leary's 1992 album No Cure For Cancer, he felt Leary had stolen his act and material. The friendship ended abruptly as a result. At least three stand-up comedians have gone on the record stating they believe Leary stole not just some of Hicks' material but his persona and attitude. As a result of this, it is claimed that after Hicks' death from pancreatic cancer, an industry joke began to circulate about Leary's transformation and subsequent success (roughly; "Question: Why is Denis Leary a star while Bill Hicks is unknown? Answer: Because there's no cure for cancer").
2000s and 2010s
More recent times have seen public rivalries between comics over the subject of joke theft. Louis C.K. has maintained a relatively quiet rivalry with Dane Cook over three bits on Cook's album, Retaliation, that allegedly bear some resemblance to three bits on CK's album Live in Houston. C.K. and Cook portray this rivalry with comedy and sincerity in an episode of C.K. series Louie. Joe Rogan, by contrast has been very open in accusing Carlos Mencia of joke theft.
To a lesser extent, George Lopez has also accused Mencia of plagiarizing his material. He also claimed he had a physical altercation with Mencia over the alleged plagiarism. Comedian Ted Sarnowski countered this claim, however, stating that he, himself, had actually written the joke and given Mencia permission to use it after Lopez had stolen it from him.
The Comedy Central show, South Park made light of Mencia's alleged joke theft in the 13th season episode, "Fishsticks" by portraying him claiming authorship of a joke written by the handicapped character Jimmy.
In 2010, Italian comic and satirist Daniele Luttazzi was accused of having plagiarised many jokes from comedians such as George Carlin, Mitch Hedberg, Eddie Izzard, Chris Rock, Bill Hicks and Robert Schimmel. But five years before those charges, Luttazzi himself told about his scheme on his personal blog: he wrote that he adds references to famous comedians' jokes to his work as a defense against the million-euro lawsuits he has to face because of his satire. (In March 2012, Luttazzi won a legal battle against La7 broadcasting company, which in 2007 abruptly closed his late show "Decameron", accusing him, among other charges, of plagiarism from Bill Hicks. Verdict: It was original satire, not plagiarism. Luttazzi got 1 million 2 hundred thousand euros as compensation.)  Luttazzi calls his ruse "the Lenny Bruce trick" after a similar trick played by his hero, Lenny Bruce. Luttazzi asked his readers to find out the original jokes. He awards a prize to anyone who finds a "nugget", i.e. a reference to famous jokes: he calls the game "treasure hunt".  Luttazzi also calls the charges "naive", explaining why those jokes are not "plagiarized", but "calqued", which is a fair use of original material. He uses a joke by Emo Philips to prove that the meaning of a joke depends on its context.  Luttazzi’s blog lists all the comedians and writers quoted in his works. 
In 2011, one of the contestants on the talent quest television program Australia's Got Talent was Jordan Paris, whose act was stand-up comedy. His act went well, the judges were impressed, and he made it through to the semi-finals. However, it was later revealed that he had plagiarised his jokes from comedians Lee Mack and Geoff Keith. The television network eventually decided to give him a chance to redeem himself and he was allowed to compete in the semi-final, provided he use his own material. Paris' effort this time was self-deprecating, joking about his plagiarism and his large teeth. The first joke went well, but the rest went downhill. It was later found out that the joke that went well – "I just sacked my two writers – Copy and Paste" – had been done in 2009 by comedian Jeffrey Ross, about Brad Garrett, at a roast of Joan Rivers. Ross had said, "This guy has two writers, their names are Cut and Paste."
In January 2012, blogger and comedian Troy Holm was ridiculed on the social networking site Facebook for stealing jokes and stories from comedian Doug Stanhope and posting them to his Blog from 2010, claiming them as his own work, including Stanhope's "Fuck someone uglier than you" routine, which was found on Stanhope's Acid Bootleg. Troy Holm also plagiarized Stanhope's story of an encounter with a transsexual prostitute nearly verbatim, substituting himself as Stanhope, and changing a few small details, causing a backlash from Stanhope's fans. This catapulted Troy Holm into an internet icon which started the "Occupy Troy Holm" Movement. Stanhope commented on the Occupy Troy Holm Facebook page that "To the few people who seem to think this is overboard...and it is...I don't think that you know the levels to which this guy has been ripping me off. He didn't take a tit-fuck joke and use it as a status update. He's been living my entire life as though it was his, changing some names and then promoting with twitters... Look at his site and most the entirety of it is me, including the comments where he uses my stuff to pass as his own conversation. And on Twitter. So who is he ripping off for that stuff that isn't mine?"
In June 2012, a live show named 'Joke Thieves' was launched in one of the longest running comedy clubs in London, Downstairs at the King's Head (Crouch End). The show was also performed as part of the Free Festival at the Edinburgh Fringe in August 2013. The London launch night was reviewed by leading U.K. comedy website, chortle.co.uk. Who called it "an undoubted hit" and "the birth of a new cult comedy night". Guest comedians are paired up on the show by the audience and go on to perform their own material in part one before stealing each other's for part two. A pilot of the Joke Thieves show was developed by the Comedy Unit and recorded for BBC2 in August 2014 at BBC Scotland's studios in Glasgow, UK. 
In other media
Joke theft is not limited to stand-up comedy. Often jokes in film and television shows are taken from comics or even other media.
Dick Cavett wrote about joke theft in his autobiography. He relayed a story about writing a bit about eating Chinese-German food and, an hour later, being hungry for power. After a few days of performing the bit, he discovered a review of Rip Taylor's show, where the joke was quoted verbatim. However, after calling Taylor to ask him to stop using the bit, he discovered that not only had Taylor never performed the bit, he had never even heard it and laughed hysterically at the joke's humor. It was then that Cavett discovered that some journalists often falsely attribute jokes to the wrong comics.
Cavett and Woody Allen often cited to each other the many instances of their jokes appearing in television shows without their permission, sometimes even falsely attributed to each other.
The experimental Pet Shop Boys film It Couldn't Happen Here and the promotional music video for their remake of the Elvis Presley song "Always on My Mind" both featured a homicidal priest, played by Joss Ackland, who performed several bits from Steven Wright's first comedy album, "I Have a Pony".
Several episodes of The Simpsons, including "Missionary: Impossible", "Treehouse of Horror XIII", and "The Italian Bob" have poked fun at Family Guy, implying that MacFarlane's show is guilty of stealing jokes and premises from the Simpsons. However, the producers of both shows have said that there is no serious feud between the two of them and their shows.
Recourse and consequences
There is, historically, very little legal recourse taken in cases of joke theft. Some comics, however, have chosen to exact their own justice. W. C. Fields reportedly paid fifty dollars to have a thieving comic's legs broken.
"You have a better chance of stopping a serial killer than a serial thief in comedy," said comedian David Brenner. "If we could protect our jokes, I'd be a retired billionaire in Europe somewhere — and what I just said is original."
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- Berle, Milton (1989) Private joke file, Introduction, p.xxiii quotation:
Of course, in the days of vaudeville, it wasn't uncommon for a performer to "borrow" a joke from another performer. Etiquette demanded only that the borrower add to the joke and make it his own. Bert Williams, a star of the Ziegfeld Follies, pilfered a story about fish and added enough laughs to turn it into a classic fifteen-minute routine. Naturally, that routine happens to be in my file.
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Woody's best lines would show up, while he was still confined to little Village clubs, on the Red Skelton show and that alleged entertainment, Laugh-In.
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The rivalry is very affectionate...
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You know, it's funny. Matt Groening and I actually have a great relationship...
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