“I’m no rat,” Capone assured the officer, as recounted in “Young Al Capone: The Untold Story of Scarface in New York, 1899-1925,” by John and William Balsamo.

Capone’s rise to power as a prohibition-era gangster roughly coincides with the use of the rodent name for someone who “secretly aids the police to apprehend criminals,” as defined in the “Dictionary of the American Underworld Lingo.” Experts date its use in the “underworld” — the abode of criminals and organized crime — to 1902, while it began to be employed by police in the 1920s, as they squeezed the underlings of gangsters and mafia bosses enriching themselves in the illicit liquor trade.

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But in 2018, “rat” is the language that emanates from the White House. President Trump applied the term on Sunday to his former attorney, Michael Cohen, who told a federal court that the president had directed him in violating campaign finance law, including by buying the silence of two women who claimed to have had affairs with Trump before he became president. A federal judge last week sentenced Cohen to three years in prison, in a development that dramatized just one of the numerous legal woes facing the president, which reach nearly every aspect of his business and political life.

In a tweet on Sunday, the president argued that law enforcement had used improper means in turning Cohen, once a loyal fixer, into an informant, or a “Rat.” In April, FBI agents raided Cohen’s Manhattan office, home and hotel room, seizing, among other material, communications between Cohen and Trump — a fact that led the president to declare attorney-client privilege “dead.”

They “BROKE INTO AN ATTORNEY’S OFFICE!” Trump raged over the weekend, even though law enforcement obtained a warrant to do so. Trump then suggested that authorities should instead have infiltrated Democratic headquarters to expose his opponent — in an apparent endorsement of the sort of tactics employed by President Richard M. Nixon in 1972.

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No shortage of ink has been spilled on the president’s bizarre Twitter locutions, from “Smocking Gun” to “very legal & very cool.” Typos, excessive capitalization, misnomers and dubious terminology have become occasions for the president’s detractors to have a laugh at his expense.

Yet, some saw in his language on Sunday something darker — a window into his legal worldview, even perhaps an unwitting acknowledgment of the highly consequential role his former fixer is now playing in assembling possible evidence against him.

Andy McCarthy, a former assistant U.S. attorney and Fox News contributor, weighed in on Twitter, informing the president of the ominous associations of his language.

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“You should stop,” he advised.

It’s not the first time Trump has labeled someone a “rat” on social media, nor is it the only instance in which his rhetoric has seemed to come from the world of the mob. In August, he took to Twitter to complain that his onetime campaign chairman, Paul Manafort, who was on trial for tax and bank fraud, was being treated worse than Capone, “Public Enemy Number One,” as Chicago labeled him in 1930. Trump has praised Manafort for not “flipping,” using language that some commentators argue makes him sound like a mafia boss.

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In a prior usage of the term “rat,” Trump seemed to suggest that the informant was worse than the person on trial. In 2012, during the campaign-corruption trial of John Edwards, the former U.S. senator and Democratic vice-presidential candidate, Trump lambasted a former Edwards aide, Andrew Young. He called the political operative a “rat,” setting the snub off in quotes. Attacking Young’s credibility was a major element of the defense, which ultimately won acquittal on one charge and a split jury on the rest.

The following year, Trump saw evidence of “Racism!” in a dispute between Shirley Huntley, a former Democratic state senator in New York, and Eric Schneiderman, then the state’s attorney general, who resigned in May after four women came forward in the New Yorker to accuse him of physical abuse. At issue was when a rat should be believed, and whether Huntley, who was handed a one-year prison sentence for fraud in 2013, could help incriminate Schneiderman in corruption — an idea dismissed by federal prosecutors.

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The real estate mogul, whose political identity at the time was built on his false claim that former president Barack Obama was not born in the United States, wrote that Huntley faced a double standard, only trusted when she was ratting on “black politicians” but not on Schneiderman.

The verb form employed by Trump in the second post also originated in the early 20th century, following closely the word’s appearance as a noun to describe “a sneaky police informant,” as true crime writer Jay Robert Nash explained in his “Dictionary of Crime.”

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In addition to this meaning, the slang word has been associated with a host of other offenses. In the 19th century, it was used in the United States to designate an unethical person, a cheat or a betrayer. A common application has been to turncoat politicians, who flee at the first sign of distress just as rats are said to desert a sinking ship. “He might have been a Peer if he had played his cards better,” observes a character in “Vanity Fair,” the 19th-century novel by William Makepeace Thackeray. “[B]ut he ratted always at the wrong time.”

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The term has had grisly associations, fitting the most unsocial behavior of the rodents. Around the same time the slang term came to signify an informant, it was also used to describe a prison inmate who robs another. In the verb form, “rat” in the 1920s also began to mean stealing from a human corpse.

“Rat” lands with specific force in the context of the workplace, where it has sometimes been used to mark a laborer who accepts wages lower than the union rate, or who toils when others are striking. This meaning, among others, was outlined by John Stephen Farmer, a British lexicographer and spiritualist, in “Slang and Its Analogues Past and Present,” first published in 1890.

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The term has had many other meanings, including a renegade, a deserter, a drunk, a spy and someone who changes views or tactics. To smell a rat is to suspect trickery or wrongdoing.

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Meanwhile, other terms for a police informer have included tout, singer, mule mouth, squealer, fink, snout, mut (short for “mutter”), snitch, stool pigeon and narc (most likely derived from the French “narquois,” meaning mocking or contemptuous).

The composite “rat fink” has an especially freighted history. Used commonly in the labor context, the smear is sometimes thought to be a rhyming reference to Pinkerton agents, who worked incognito to expose union organizers in the late 19th and early 20th centuries. It has also been applied to replacement workers hauled in to break a strike, also called “scabs.”

Merriam-Webster dates its use in the context of informants to 1964, one year after the first appearance of the hot-rod character “Rat Fink,” created by Ed Roth, who is known to fans as “Big Daddy.” Rendered in a grotesque green or gray, with bulging eyes, Rat Fink is the anti-Mickey Mouse. The Walt Disney character is mischievous and winsome, entirely lovable. And quite unlike a rat.