For years, Ricky Stanicky has been the ultimate scapegoat for whatever mischief best buds Dean, JT and Wes cooked up. When they nearly burned down a neighbor’s house as kids, Ricky took the blame. That’s what friends are for, right? Except Ricky’s not real. The trio invented him in their teens, and they’ve been pinning things on him ever since. Now, with their significant others starting to get suspicious, the three amigos decide to hire an actor to play their imaginary friend.

Directed by Peter Farrelly, “Ricky Stanicky” is a raunchy, R-rated return to form for the “Dumb and Dumber” co-director: an unapologetically lowbrow buddy movie, featuring the funniest performance yet from up-for-anything wrestler John Cena in the title role. Remember, Peter is the Farrelly brother who went off and made best picture-winning “Green Book,” followed by the (largely unseen) Vietnam drama “The Greatest Beer Run” with Zac Efron. For a moment, it looked like he might have gone respectable, but “Ricky Stanicky” shows that he simply can’t resist the lure of dick jokes.

Headed straight to streaming, the Prime Video release is clearly intended for audiences who feel the same way: bros, obviously, but also women who grew up on the “High School Musical” movies and like the idea of seeing Efron poke fun at his image. As grown-up Dean, he’s miles away from his Disney Channel roots, whether lying to his girlfriend (Lex Scott Davis) or trying to drug a rival into a ketamine-induced K-hole.

Over nearly a quarter-century, Dean and expectant father JT (Andrew Santino) and gay best friend Wes (Jermaine Fowler) have perfected an elaborate system for duping their partners, employers and parents whenever they need an alibi for a boys’ night. The routine involves strict rules, a “bible” full of all their Ricky-related shenanigans and regular postings to a phantom Instagram account, which gives people the impression that the once-rowdy Ricky is now a world-saving humanitarian. But their circle is starting to get suspicious about their nonexistent friend (who’s supposedly battling his second round of testicular cancer), since no one has ever seen the guy.

Enter Cena as a washed-up actor they meet in Atlantic City, where he sings “jerk jams” (familiar tunes Yankovic’d into wildly inappropriate parodies) under the stage name “Rock Hard” Rod. Those off-color cover versions make for one of the more outrageous bits in a film that rarely takes its ostensibly offensive routines far enough. By the time Rod hits rock bottom — sprawled in the alley behind his club, dressed in full Britney Spears “Baby One More Time” drag, trying to slurp spilled whiskey off dirty cardboard — Cena has proven himself this movie’s MVP.

While Cena delivered good-sport support in a couple of Judd Apatow productions (“Trainwreck” and “Blockers”), he’s had worse luck choosing leading roles. That makes carrying a Farrelly comedy quite the coup for him. In theory, practically any actor could have played Ricky Stanicky — no one’s ever seen the guy, after all — and yet, Cena makes it impossible to imagine another person in the part. He’s game to go big, which fits Rod’s frustrated-actor persona, while also able to play vulnerable and sincere.

However extreme “There’s Something About Mary” co-director Farrelly likes to play it, he’s always been a softie at heart. So, while he’s willing to mine every last joke from a ceremonial bris (where guest of honor Ricky becomes the replacement surgeon after the mohel passes out mid-circumcision), the movie doesn’t really intend to offend. If anything, it’s yet another celebration of what makes us all unique, as represented by the sort of all-inclusive casting sensibility that sets the Farrellys’ films apart.

In this case, there’s a gay man with crutches and a millionaire of short stature mixed into the ensemble, both (rightly) treated as no big deal. Meanwhile, there for a laugh are two follically challenged individuals. First is Dean and TJ’s investment banker boss (William H. Macy), who’s just gotten a hair transplant — or “seedlings,” as Ricky refers to the fresh plugs. In the film’s most memorable gag, the poor guy punctuates his pep talks with highly embarrassing hand gestures (making the phenomenon “air dicking” this comedy’s most likely takeaway). On the other end of the hair spectrum is Carly, whom the guys derisively refer to as “Cousin Itt” for her ridiculously long mane. This Rapunzel-like weirdo (adorably played by the director’s daughter, Apple Farrelly) has been following Ricky’s Instagram profile, and can’t wait to meet the guy Dean says has been saving the world with Bono.

It’s easy to see where this is headed, since the movie has nothing but affection for outcasts. In fact, apart from the near-miss bris debacle, the potentially comic situations never go far enough. That’s clear from the opening example, where the boys blamed Ricky Stanicky for setting a fire that stops short of doing any irreversible harm. As adults, the trio are despicable characters who’ve spent decades lying to their loved ones. Santino doesn’t hold back in playing TJ as a jerk, but Efron and Fowler hedge their performances, since Farrelly is counting on them to be easily forgiven once the whole scam comes to light.

The ending doesn’t really work, as it tries to redeem the rest of the ensemble, who’ve been treated like dummies too naive to suspect the gaping cracks in the Ricky Stanicky story. More successful is the idea that Dean, TJ and Wes stunted their own maturity by clinging to this lie for so long (the trio’s performances are revealing variations on the man-child syndrome). Ricky Stanicky may have been a great excuse for their worst behavior, but he kept them from growing up. Consider this a lesson in what “real” friendship means.

”Ricky Stanicky” streams exclusively on Prime Video starting March 7.