Three friends get more than they bargained for when they hire a low-rent celebrity impersonator to pose as the imaginary buddy who has served as their scapegoat since childhood.

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.

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.

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.

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.