What Does Trump mean when he calls Cohen a 'Rat'? Trump's attack on Cohen was just the latest example of his using 'mob' talk.

"Never rat on your friends, and always keep your mouth shut."

That's a famous line from Robert Di Niro's organized crime boss character in the movie "Goodfellas."

But does that "mobster rule" apply in politics? President Donald Trump seems to think so.

Over the weekend, the president attacked his former personal attorney, Michael Cohen, in a tweet, calling him a "Rat." Cohen, who has pleaded guilty to campaign finance and other violations, claimed that Trump "directed" him to make hush money payments to two women out of concern about how the allegations of the affairs would affect the 2016 election if they became public.

Trump has denied the affair allegations.

A rat, commonly known as an FBI informant, often refers someone from an organized crime family who has gotten arrested and cooperated with the federal government to provide information about a larger crime or crime family aka an informant, according to Merriam-Webster.

When asked by ABC News if Cohen being branded as a "rat" would be dangerous, Michael J. Stern, a former federal prosecutor said it could be.

"Trump is not disclosing anything the prison population will not already know, given Cohen's very public disclosure of his cooperation. In general, cooperators are often subject to retribution, by the people they cooperate against and by prisoners who don't like the idea of people cooperating with the government. One nuance, in this case, is that there will be prisoners who support Trump and his branding Cohen a "rat" may be perceived as a call for them to punish Cohen," he said.

And of course, there may be prisoners who don’t support the president who might view the term as a compliment.

People close to Michael Cohen say the term “rat” is not accurate and they believe it is outrageous that the nation’s chief executive would use such a term and inject himself into the case in this manner.

ABC News reached out to the White House for comment but got no immediate response.

Former FBI director James Comey, on Capitol Hill Monday to face questioning from House Republicans over how he handled the Clinton email and Russia investigations before Trump fired him in May 2017, echoed his previous criticism voiced in an interview with ABC Chief Anchor George Stephanopolous.

"It undermines the rule of law, " Comey told reporters. "This is the President of the United States calling a witness who has cooperated with his own Justice Department a rat. Say that again to yourself at home and remind yourself where we have ended up."

In the ABC News interview this spring, Comey said he was alarmed that Trump sounded like a mob boss.

"I'm not trying to … suggest that President Trump is out breaking legs and shaking down shopkeepers. But instead, what I'm talking about is that leadership culture constantly comes back to me when I think about my experience with the Trump administration,” Comey said.

Jerry Tillinghast, a former mob enforcer in Providence, Rhode Island, and author of the book "Choices: You Make 'em You Own 'em: The Jerry Tillinghast Story," said a rat can take different forms such as a rat on the street and a rat in prison.

The most common type, according to Tillinghast, is someone who committed a crime with you and then turns around and tells the cops about it. He adds that, in prison, a "rat" is the worst name you can give someone.

They are also detested by members of organized crime.

Trump's use of the word 'rat' is not the first time he's referenced a popular mob term. He's previously talked about Cohen "flipping," a common practice in which prosecutors offer criminal defendants reduced punishment if they testify against another criminal.

"It's called flipping and it almost ought to be illegal," Trump said in an interview with Fox News in August.

Sen. Chris Coons, a Democrat from Delaware, slammed the president for his attacks, likening his language to that of a mob boss.

“The idea that the FBI broke into his attorney's office runs right up against the foundation of our law, which is the FBI was executing a duly authorized warrant,” Coons said in an interview on CNN’s “New Day" Monday morning.

“They were executing a warrant issued with the approval of a judge. This is part of how investigations work. His use of the term ‘rat’ for Michael Cohen and mischaracterizing this as a break-in to his attorney's office frankly makes him sound more like a mob boss than President of the United States.”

Former Attorney-In-Charge of the Federal Organized Crime Strike Force in New York Edward McDonald, who has prosecuted organized crime members, said that a rat is "someone who turns on friends or criminal accomplices and provides criminal evidence against him."

In the majority of public corruption cases, anyone who is arrested could be a potential rat, McDonald also appeared in "Goodfellas", added.

Stern said that calling Cohen a rat undermines the presidents' own Justice Department's work.

"The president’s reference to cooperating witnesses as “rats,” undercuts the work of his own Justice Department that uses cooperators to prosecute some of the most dangerous criminals in the country. The president’s comments makes current federal prosecutors break out in a cold sweat, former federal prosecutors glad they left the Justice Department, and dead federal prosecutors roll over in their graves," Stern told ABC News.

Stern went on to say that President Trump acts like a mob boss.

"Mr. Trump is fond of referring to people who look like the classic character of the job they perform as coming from “central casting.” Referring to a cooperating witness as a “rat,” is central casting of a mob boss," Stern continued.

It could be likely that the President took the language from Matt Drudge who has been critical of Michael Cohen - and this is not the first time Trump has used the language. He referred to Bob Gates as a "rat" in a 2014 tweet.

The history of the word can be traced back to 1963 when Joseph Valachi testified in front of the Senate Subcommittee on Investigations - and not only acknowledged La Cosa Nostra's existence but described the inner workings of the Mafia in detail. This was considered a big betrayal. These hearings in front of the Senate Subcommittee were known as the Valachi Hearings.

Other infamous federal informants, or "rats," include Sammy "The Bull" Gravano, who became a federal informant and helped take down John Gotti and Whitey Bulger. Bulger, who was recently killed in prison, was an FBI informant for years before his capture and trial.