Zac Efron, John Cena in Peter Farrelly Comedy

After showing his serious side with Green Book, the most derided Best Picture Oscar winner since Crash, then taking a critical drubbing with the Zac Efron vehicle The Greatest Beer Run Ever, Peter Farrelly returns to the kind of unapologetically silly comedy he and his brother Bobby parlayed into box office gold in the ‘90s. At least in theory. Efron gets stuck with another slipshod script in Ricky Stanicky, a sad demonstration that what was once considered outrageous, transgressive and anarchic now just seems crass, tired and witless.

Streaming on Prime from March 7, the movie might tap into the nostalgia of audiences raised on the Farrelly brand of goofy raunch, gross-out laughs, slapstick, escalating chaos and sticky sentiment. The virtual extinction of the midbudget R-rated studio comedies that thrived in the ‘90s might also help, as the success of Anyone But You recently showed.

But even if Ricky Stanicky never met a penis joke it didn’t find side-splitting, the caution required to serve up crude humor without offending 21st century sensitivities often makes it feel as awkward as most of the cast’s comic timing.

The title character is an invention, a name hurriedly scrawled on a discarded jacket when three best friends flee the scene of a Halloween prank that gets out of control. An animated opening credits sequence shows how that imaginary friend, Ricky Stanicky, became their convenient cover for all kinds of misbehavior through their school years and into adulthood, two decades later. “Stanicky did it” is their mantra.

It took a team of six writers to come up with the dumb script, and yet only the flimsiest explanation is provided for the fact that no one — not school authorities, cops, parents or current partners — appears to have seriously questioned the existence of this phantom friend or wondered why they’ve never met him.

The three amigos, Dean (Efron), JT (Andrew Santino) and Wes (Jermaine Fowler), regularly spin the Ricky excuse to help them shirk family obligations and catch sports games, concerts and other recreational pursuits. The latest is a weekend in Atlantic City, made possible by ditching the baby shower for the child JT and his wife Susan (Anja Savcic) are expecting. Never mind that JT is so obsessively involved in every aspect of their impending parenthood that he calls himself “Daddy Doula.” Nobody should expect consistency from this movie.

In Atlantic City, the guys meet “Rock Hard” Rod (John Cena), an alcoholic failed actor now billing himself as “South Jersey’s Premier X-Rated Rock ‘n’ Roll Impersonator.” His act consists of fully costumed “jizz jams,” hits of artists including Devo, Alice Cooper, Billy Idol and Peter Frampton with lyrics reworked around the theme of masturbation. Cena also sports Britney Spears “Baby One More Time” drag, but mercifully, we’re spared the song.

When Susan prematurely goes into labor, Dean & Co. cut their Atlantic City escape short. They head back to Providence, Rhode Island, where an uncanny number of people speak with Australian accents, an anomaly explained only by the knowledge that the production was shot in Melbourne.

While the guys have a whole bible of elaborate Stanicky factoids and an Instagram feed to chart the fictitious friend’s movements, they run out of excuses when it comes time for the newborn boychild’s bris, which feels like the ne plus ultra of Farrelly gag set-ups. Seriously, this film has so many penis jokes you feel like you’re being clobbered with them.

Dean’s bright idea to hire Rod to play Stanicky works beyond their wildest expectations. He takes the role very seriously, sobering up and even stepping in for the mohel after a ketamine mishap — don’t ask. He charms everyone, including Dean and JT’s financial investment firm boss, Ted Summerhayes (William H. Macy), who swiftly puts “Ricky” on the payroll. By that point, any tenuous connection the screenplay has to the real world is abandoned.

Dean and JT go into desperate damage-control mode, but Ricky/Rod swiftly endears himself to his new boss, notably by pointing out to Ted his unwitting habit of making embarrassing hand gestures while addressing the boardroom. Ricky calls these moves “air-dicking,” and no, I’m not going to explain.

In what would probably have been the Jim Carrey role back in the day, the always likeable Cena appears to be having a blast as a big, dopey screw-up who gets an unlikely shot at redemption via his new identity. But even Farrelly’s usual sweet-natured generosity toward his characters can’t sell the contorted way the ruse is inevitably exposed and instantly forgiven. That applies to Dean’s savvy local TV producer partner Erin (Lex Scott Davis), as well as to Ted and his prospective merger associates.

Efron’s appealing sincerity goes some way toward making us believe there’s something at stake for his character, even if the actor pretty much walks through his scenes, as if he’s aware this kind of material is past its prime. The stab at providing background texture by alluding to Dean’s unhappy childhood — something he and Rod have in common — amounts to nothing. Same goes for the indications that he doesn’t want children, whereas Erin might feel differently.

Santino and Fowler do what’s required, though the decision to make Wes gay makes sense only in terms of representation. Sure, Farrelly deserves points for making the character’s sexuality no big deal to his lifelong friends, though Wes is like a queer person created by filmmakers who have never met a queer person. A relationship crisis with his partner Keith (Daniel Monks) evaporates as soon as it’s mentioned.

That’s also because the movie has a quasi-magical force for good in the lovable lug who comes on like a loose cannon, threatening disaster at every turn, but somehow, with guilelessness and honesty, always manages to emerge smelling like roses. Unfortunately, Ricky Stanicky smells more like mothballs.