James Mulvaney is an adjunct professor at John Jay College of Criminal Justice and served as deputy commissioner of the New York State Division of Human Rights.

James Mulvaney, the author’s great grandfather, as a Brooklyn police patrolman in the 1890s. (Family photo)

The use of the derogatory phrase “paddy wagon” wasn’t the worst thing President Trump said last week. But it shouldn’t get lost amid the litany of objectionable utterances.

In a speech on Long Island Friday, Trump bellowed a series of nonspecific pronouncements regarding his administration’s strategy to combat the deadly street gang MS-13. He provoked the ire of police chiefs across the country with his statements condoning the roughing up of arrestees. “When you see these thugs being thrown into the back of a paddy wagon — you just see them thrown in, rough — I said, please don’t be too nice,” Trump riffed. “Like when you guys put somebody in the car and you’re protecting their head, you know, the way you put their hand over? Like, don’t hit their head and they’ve just killed somebody — don’t hit their head. I said, you can take the hand away, okay?”

As a teacher of police science, I too feel strongly that prisoner mistreatment is a counterproductive crime-fighting tool and appropriately deemed a felony. The topic was particularly irresponsible when addressing a police department whose former commissioner is serving 46 months in federal prison for roughing up a prisoner.

But I was also put off by Trump’s description of police rounding up gang members into a “paddy wagon.” To many Irish Americans like myself, the phrase is insulting; it should not be used in polite discourse.

Paddy, of course, borrows from the pet form of the common Irish name Patrick and has long been deployed as a slur. The origin of “paddy wagon,” though, is unclear.

According to one popular account, the phrase dates to the mid-19th-century when U.S. cities — notably New York and Boston — were flooded with Irish immigrants escaping the catastrophic food shortage at home. The brand-new New York City Police Department (first called the Municipals, later the Metropolitans) was dispatched to round up the Irish on suspicion of drunkenness or to dragoon them into the Union Army. The police back then delivered arrestees to stations in horse-drawn vans called “Black Marias,” named after a famous racehorse and referring both to the signature color and speed. But the vans came to be known as “Paddy wagons” because of the ethnicity of their cargo. Social commentator William J. Stern cited this account in an essay for City Journal in 1997. “Over half the people arrested in New York in the 1840s and 1850s were Irish,” Stern wrote, “so that police vans were dubbed ‘paddy wagons’ and episodes of mob violence in the streets were called ‘donnybrooks,’ after a town in Ireland.”

Etymologists are somewhat skeptical of that theory. In newspapers and other sources from the 1800s, “paddy wagon” typically refers to a wheelbarrow. Merriam-Webster says that “paddy wagon,” meaning police vehicle, came into use in 1909. By then, the Irish had become a significant part of law enforcement. Nearly 70 percent of the New York police force was made up of Irish immigrants or first generation Irish Americans, according to author Richard Zacks. So it may be that “paddy wagon” had less to do with the prisoners thrown into the back than with the police driving in front. Indeed, by the 1960s, civil rights protesters had adopted “paddies” as a generic term for the police.

The Rev. Martin Luther King Jr.’s “I’ve Been to the Mountaintop” speech refers specifically to “paddy wagons.”

“We would be thrown in the paddy wagons, and sometimes we were stacked in there like sardines in a can,” King recounted. “And they would throw us in, and old Bull would say, ‘Take ’em off,’ and they did; we would just go in the paddy wagon singing, ‘We Shall Overcome.’ ”

I certainly don’t think King intended to slight Irish Americans. And Trump probably didn’t either. But that doesn’t take the sting out of the phrase when I hear it.

My great grandfather, a cop who first walked a beat in Brooklyn and later for the NYPD, told me of early job searches at the turn of the last century and the widespread presence of signs saying, “Irish and dogs need not apply.” His son, my grandfather, served as editor of the Cornell Law Review in 1953 and was rejected by every white-shoe law firm in Gotham, apparently because of his last name. As a foreign correspondent in Belfast in the 1980s, I was repeatedly tossed into police vans — presumed guilty for having the temerity to live in a predominantly Catholic neighborhood. Slaps of nightsticks to the shins and kicks to the ribs were accompanied by a variety of vile phrases ending with the word “paddy.”

Two years ago, I complained to the New York Times about the appearance of “paddy wagon” in a crossword. Puzzle editor Will Shortz dismissed my objections, writing: “The Irish are not a group that’s discriminated against in the U.S.”

That’s generally true. But each use of the phrase “paddy wagon” evokes a time when they were.