Forget Elvis. Give no more thought to accusing Eminem.

“White Boy Rick” might be the ultimate cultural appropriator.

He adopted street argot — abandoning verbs almost entirely — embraced black slang and took on an African American-influenced wardrobe long before Marshall Mathers learned to rhyme.

White Boy Rick trafficked in illegal firearms and moved into crack cocaine when it proved to be the more lucrative business in a dead end neighborhood in a fast-decaying Detroit, where hope died in the ’80s.

A baby daddy at 16, with a junkie for a sister, he was an ethnic outlier, an early adapter of the most negative associations of a culture that wasn’t his but a class — poor and desperate — that was.

Warned that there was a difference between the attention black teens and men earn from the police, Rick was the white boy the Feds and local cops swarmed over. And further warned that there’s a BIG difference between “White Time” and “Black Time” when it comes to prison sentences, he wound up serving “Black Time.”

If his story seems familiar, we’ve seen it on big screens and small ones for decades, a cultural cliche, the most pervasive inner city African American stereotype there is. The white boy lived it.

The film based on this true story, directed by Yann Demange (”71″) is by turns swaggering and sentimental, cocksure and callow, the many moods of Rick himself, played with as word-slurring, naive bravado by screen newcomer Richie Merritt.

Even at 14 in 1984, Rick can spot a “fake” AK-47 (Egyptian made) at a gun show, and use that information to score himself really good deal on it. Rick Wershe Sr. (Matthew McConaughey, in a nuanced turn) taught him well.

But to what end? His wife left him, and she “left YOU too,” he reminds his kids. Rick’s already hustling, a stranger at school. Daughter Dawn (Bel Powley of “A Royal Night Out” and “Diary of a Teenage Girl,” stunningly transformed here) is a junkie, sleeping with any guy who can get her what she craves.

They might live right across the street from Grandma (Piper Laurie) and Grandpa (Bruce Dern). But the whole neighborhood around them’s gone to ruin. Gun dealing Dad is the only one to realize they’re not just surrounded by “lowlifes,” they’re “lowlifes” themselves.

He talks big and butch and dreams of getting into the Next Big Thing (a video store). Rick Jr.? He’s hanging with his friends, all of whom are black, with best friend Boo (RJ Cyler of “Me and Earl and the Dying Girl”) a member of the Curry crime family.

To Rick, it’s the most natural thing in the world to roll into Curry HQ and hustle big boss Lil’Man (a smart and mercurial Jonathan Majors) some of those Egyptian AK-47s, “upselling” them silencers that gunsmith Dad machines in their basement.

But the kid quickly finds himself under the FBI microscope, strong-armed by agents (Jennifer Jason Leigh, Rory Cochrane) into cozying up to the Currys, dealing drugs with them, attending Lil’Man’s mayor’s mansion wedding to the most gorgeous woman at the Skate and Roll rink, Cathy (Taylour Paige).

Yeah, the Feds are after the Currys, corrupt cops (“This Detroit, boy. If you ain’t on the take, you get took!”) and crooks high up in the administration of Mayor Coleman Young. They’re so desperate they’re willing to use a 14 year-old boy as an informant.

Not that Rick’s a snitch. He’s just doing what they say, and as he does, he gets deeper and deeper into the mob’s business (Eddie Marsan plays another Miami-based drug supplier, just as he did in “Miami Vice”) and social life (a weekend in Vegas to attend the ’85 Hearns/Hagler title fight.).

The son of a corners-cutting gun dealer hasn’t learned much about morality, and Rick is quick to pull a pistol and even fire it in anger, requiring an FBI bailout. His first taste of Curry violence rattles him. But he can’t see anything but the dire straits they’re in now and how “everything just gets worse.”

Merritt is great at conveying the insensate impulsiveness of youth. Of course he doesn’t wear a condom. Of course he’s “brave.” He doesn’t consider consequences, and only slowly awakens to the murderous mayhem his death-dealer Dad is putting on the street, and the utter amorality of his own decision to get into selling drugs.

McConaughey’s Rick Sr. is living a long, dark night of the soul — a drug-addicted daughter who flees him, cops who muscle his kid, mobsters willing to kill any and all of them if they get out of line and the grim realization that when he chose his lowlife line of work, he made the world more violent and worse for everybody in it. Rick Sr.’s darkest moment is realizing he’s not moral enough to rise above gun selling, not “hard” or brave or connected or smart enough to extract Jr. or anybody else from their predicament.

“White Boy Rick” starts out as playful as its title, teeters into sentiment as Rick takes on responsibilities with both his “families,” both of which he betrays, and drops into jaunty here and there as he absent-mindedly bargains with cops and killers and hits the street corner to make his and his family’s fortune.

“I’m lookin’ for a gun. Grandma keeps hidin’ mine!”

The script scores points about the racial injustice of drug laws of the era and plunges into moralizing in a third act that might turn maudlin, if we’d allowed ourselves to care that much about anybody in this sordid circle of sin and vice, “desperate” or not.

It’s not “Blow” or “American Gangster” or “American Made” even, not on that level of sobering (if sometimes comical) morality tale. But “White Boy Rick” still makes for a blunt reminder of just how low we all sank during the “Just say no” ’80s, when the only people punished for not saying “No” were co-enabled into saying “Yes,” and faced “Black Time” for doing it.

MPAA Rating: R for language throughout, drug content, violence, some sexual references, and brief nudity

Cast: Richie Merritt, Matthew McConaughey, Taylour Paige, Bel Powley, Jennifer Jason Leigh, RJ Cyler, Rory Cochrane, Piper Laurie, Bruce Dern, Eddie Marsan

Credits:Directed by Yann Demange , script by Andy Weiss, Logan Miller, Noah Miller. A Sony/Columbia release.

Running time: 1:50