The movie “Mack & Rita” — which adds grandma chic to two things no one needs on screen like lazy filmmaking and a tired old concept — can be distilled into one word: cringe.

Virtually no one associated with this film should be congratulated in any way, having ruptured any bridges between Hollywood and senior citizens or for the shocking misuse of Diane Keaton’s considerable skills.

“Mack & Rita,” which opens Friday in cinemas, takes the traditional body-swap concept — think “Big,” “Freaky Friday,” “13 Going on 30” and even “Jumanji” — and makes it worse, much worse.

This time, it’s a 30-year-old author with a perfect apartment and a little dog who goes to a sketchy past-life regression treatment and winds up as a 70-year-old hipster granny, played by Keaton, who is given such a poor role that she mostly just flails about in a series of flat, pointless scenes.

Written by Madeline Walter and Paul Welsh, it’s got the sort of science-supernatural plot and vibe from ’80s films like “Weird Science” or “Back to the Future” and adds a rom-com, mismatched love interest to spice up a failing premise. It wants us to learn to embrace age gracefully and let everyone be happy with who they are. But first there must be a lot of anti-ageist physical comedy.

It starts with our author Mack (played nicely by Elizabeth Lail) who is struggling with a second book and must do lowly Instagram posts for cash. She feels out of step with her fashionable Los Angeles friends — among them a memorably good Taylour Paige — and though she’s styled to slay, her inner soul is pooped and just wants to sit down holding a red wine with an ice cube.

If you were wondering what was going to happen in this film, a voice over helpfully announces: “I grew up always feeling like I was an older woman trapped in the body of a little girl.” Presto, that becomes reality.

Keaton takes over about the 20-minute mark as Rita, but she’s usually the butt of jokes, not the creator. Director Katie Aselton not only misuses the Oscar-winner, but her scenes often don’t end as much as they are abandoned or peter out awkwardly.

This is a movie where if there’s a swimming pool, Keaton will stumble into it. If there’s wine, she’s gonna guzzle it and say something inappropriate. The film’s lowest point is set at a Pilates studio, where a montage of Keaton using resistance straps in utterly unexpected ways ends with a face plant.

“This is really a nightmare. My hair’s different. It’s like really weird,” she says. “It feels like it’s well-coated wire or something.” Then, after losing four decades in 10 minutes, she’s shown happily trying on clothes.

In some ways, we must credit Keaton herself partly with the making of this monstrosity long before it was a script. She has been hailed as inspiration for the so-called “coastal grandma” look that is apparently all the rage among the cardigan-and-linen pants-wearing crowd. But she didn’t deserve this horror show, did she?

Though the young author inside the grandma initially tries to fight her instant aging with various procedures and youthful treatments — and a terrible scene where she’s high on ‘shrooms — she comes to realize something about age: “It’s not an insult. It’s just where I’m at: I’m old.”

So why has this film seemed like such an insult?

There was some outcry recently about “Batgirl,” a film that was fully made but never released due to creative and financial calculations. Why that fate wasn’t destined for “Mack & Rita” is a bafflement.

“Mack & Rita,” a Gravitas Ventures release, is rated PG-13 for “some drug use, sexual references, and language.” Running time: 94 minutes. No stars out of four.