Pissgate is the term given to a series of documents published by BuzzFeed that allege Donald Trump has deep ties to Russia and assertions in intel that supposedly could be used to in blackmail him. BuzzFeed states that the claims made in the documents are unverified. Trump and Russia have denied them.
The dossier was obtained by Buzzfeed writer Ken Bensinger through a former British intelligence official. The documents had been circulating around elected officials, journalists, and the intelligence community for several weeks, but have not been released because they are wholly unverified. 
Content of Dossier
The dossier alleges that Russia ran an operation to cultivate Trump for the past 5 years in order to take advantage of him when he takes a position of power. One part of the operation attempted to lure Trump into lucrative real estate deals for the upcoming 2018 World Cup, but he declined them.
The operation allegedly received compromising material about Trump and his sexual perversions. The dossier says that Trump booked the Presidential Suite at the Ritz-Carlton hotel in Moscow that he knew Barack Obama and Michelle Obama stayed in. The document continues on to say that to spite them, he hired a number of prostitutes to perform 'golden showers' (urination) on the bed in front of him. The room is said to be compromised by the FSB with microphones and concealed cameras. Penthouse Magazine is offering $1 million dollars for the exclusive rights to Donald Trump's golden showers.
Trump was looking for real estate opportunities in St. Petersburg and Moscow but did not find anything. He settled for prostitutes instead. There is a repeated word in this section of the document with the word "had" repeated twice. Trump's lawyer, Michael Cohen, allegedly had meetings in Prague with Kremlin officials. Cohen tweeted a picture of his passport saying that he has never been to Prague. The dossier speaks of a close relationship between the Alpha Group and Vladimir Putin." Alpha is spelled wrong and is supposed to be spelled "Alfa."
When asked to comment about the documents, Trump lawyer Michael Cohen unequivocally denied the claims, stating that "It's so ridiculous on so many levels. Clearly the person who created this did so from their imagination or did so hoping that the liberal media would run with this fake story for whatever rationale they might have." 
Editor-in-Chief Ben Smith tweeted an email he sent to staff on his decision to publish the story because he wanted "Americans to make up their own minds about the allegations about Trump that might have circulated in the highest levels of government."
Journalist Julia Ioffe was presented with the dossier about Trump but declined to publish the story because it was impossible to verify. Adam Goldman of the New York Times criticized both CNN and BuzzFeed on Twitter for their publishing practices about the story.
Possible 4chan Involvement in Fabricating the Documents
A post on 4chan on January 10th claims that the dossier published by BuzzFeed was an elaborate hoax.   The elaborate troll was allegedly started back in October when a post on 4chan on October 26 stated: “mfw managed to convince CTR and certain (((journalists))) on Twitter there'll be an October surprise on Trump this Friday.” On October 29th, another 4chan post claims that someone with a fake Hillbot account convinced Rick Wilson that he had a damaging tape concerning Trump and preyed on Wilson's wishful thinking. On November 1st, another post said “So they took what I told Rick Wilson and added a Russian spy angle to it. They still believe it. Guys, they're truly fucking desperate - there's no remaining Trump scandal that's credible.” 
Wilson, who managed Evan McMullin's presidential campaign, allegedly looked for months for a news organization to publish the story and hinted that he was up to something with his tweets. In October, Wilson said that he had a story that could "end the race." In addition, Wilson allegedly went to the CIA with the dossier who then reported the unverified reports to Trump and Obama.
Wilson has denied that his source was /pol/ or that he was Buzzfeed's source.