Isiah Whitlock Jr. attends the New York premiere of “BlacKkKlansman” on July 30. (Michael Loccisano/Getty Images)

Strangers frequently walk up to Isiah Whitlock Jr. and swear at him.

“It’s been weird at times,” the veteran actor says. Then, “you have what I call the cowards . . . They’ll wait until they’re maybe a block away, and then I’ll hear it. I guess they feel like they’ve gotten out of range of getting punched, or something like that. I would never do that.”

He would not because, aside from not being that aggressive of a person, he expects the interactions. Whitlock is most known for his catchphrase: a certain four-letter profanity that begins with “sh,” the pronunciation of its sole vowel dragged out until that letter count nears 15. It is generally associated with his character Clay Davis, a corrupt Maryland state senator who figures into several seasons of “The Wire,” but it has also made guest appearances in Spike Lee movies like “She Hate Me.”

“It just kind of stuck,” Whitlock explains.

He says it again in Lee’s new film “BlacKkKlansman,” which centers on an undercover black cop named Ron Stallworth (John David Washington) who infiltrates the Ku Klux Klan in the late 1970s. Whitlock plays a recruiter for the Colorado Springs Police Department and asks Ron what he would do if another cop called him the n-word. Ron’s initial response is to be surprised at the prospect of that happening, a display of naivete that draws out Whitlock’s catchphrase.

The line is played for laughs, as such pop culture references tend to be. But Whitlock says he tries to be careful about when, where and how he uses the word. In “BlacKkKlansman,” for example, it “helped set the tone with a little bit of comedy, a little bit of seriousness.”

It also made sense for this movie in particular, given Lee is partially responsible for the catchphrase. Despite repeated use on “The Wire,” Whitlock says he first said the word while on-screen with Edward Norton in “The 25th Hour,” released in December 2002, the same year the HBO series premiered.

The timeline is a little muddled here — “The Wire” writer-producer George Pelecanos says in Jonathan Abrams’s book “All the Pieces Matter” that creator David Simon originally wrote the catchphrase into Clay Davis scenes because “a lot of black guys” say the word like that. Whitlock clarifies in the book that he barely appeared in the first two seasons, and Simon’s assistant once told critic Alan Sepinwall that the earliest she could find the word in old scripts was a Season 3 episode that aired in October 2004.

Simon told Sepinwall that the first time he saw “The 25th Hour” was right before Season 4. “Maybe it started with [Whitlock], but I think we heard the Southern drawl in his tone and went with it,” he said. Whitlock was raised in Indiana by parents originally from Tennessee, and he describes his accent to The Post as a mix of the two: “Sometimes things kind of land, like, high in my nose,” he says. “It gets a little bit of a twang to it.”

(The Post has put in a request for comment from Simon.)

Regardless, Whitlock holds that Lee latched onto the word after the actor said it that way during his audition for “The 25th Hour.”

“When I showed up to set, I wanted to do something else,” Whitlock says. “Spike said, ‘No, you do it now. This is where you do it.’”

Several years passed before Whitlock understood the fascination with his pronunciation of the word. He had an uncle who used to say it two or three times a day, so it did not seem like anything out of the ordinary: “You couldn’t ask him a question or anything like that without him doing that. I got a very good lesson in how to say it.”

Others may tire of such requests — please do not ask a weary Chadwick Boseman to do the Wakanda salute, even if he insists he is not sick of it — but Whitlock fully embraces them. Four years ago, he created the Whitlock Academy to help “serious actors” learn how to say the phrase, which he now uses on Twitter to react to news about President Trump. The academy’s social media accounts serve as way to promote Isiah Whitlock Jr. Talking Bobbleheads, which were born out of a Kickstarter campaign he launched in 2015. (He even sells signed collectible versions, if you have $75 to spare.)

“You can have me at home with you,” Whitlock says, adding of the phrase’s longevity, “It’s taken on a life of its own, but it’s been good.”

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