Welcome home, Will Ferrell. It’s nice to have you back, and not as, say, a sex-addicted curling champ from Liechtenstein with a testicle that hasn’t descended and an appetite for Funyuns.

No, Step Brothers, the most uproarious film Ferrell has done since 2004, proves a defensive rebound from the dismal Semi-Pro — Ferrell’s last sports comedy. (Pray that’s “last” as in “final.”)

Also, it closes co-writer Ferrell and co-writer / director Adam McKay’s unofficial “modern American man” cycle. Anchorman: The Legend of Ron Burgundy brilliantly lampooned workplace sexism, and Talladega Nights: The Ballad of Ricky Bobby was a so-so spoof of stockcar machismo.

With as much meaning as a silly, scatological movie can muster, Step Brothers mocks the measurement of male maturity in a world where many “kids” have moved back in with parents.

Yes, the path of a modern-day man-child is a well-trodden trail for ubiquitous producer Judd Apatow (Knocked Up, The 40-Year-Old Virgin), and Step Brothers feels like the exhaustion point of that idea. (It helps that it’s short, not pushing Apatow’s usual 120-minute mark.)

Yet Brothers extends, to feature length, the airy viral-video jabs on Ferrell and McKay’s Funny Or Die Web site, with surreal absurdity and goofy comic commitment from Ferrell and co-star John C. Reilly (who also co-created the storyline). The pair, who made a decent comedy team in Talladega, truly soars when not tethered to slavish NASCAR tie-ins.

Ferrell and Reilly are Brennan and Dale, unemployed Californian fortysomethings living with their single parents. Their developments aren’t arrested. They’re serving life sentences in Chino.

Breakfast and self-love are morning routine for Brennan. He’s been called the “songbird of his generation,” but he hasn’t sung a note since a traumatic Pirates of Penzance incident in high school triggered by his viciously overachieving younger brother, Derek (Adam Scott).

In his junior year, Dale dropped out of college to join the family business. Too bad it’s medicine. Now, he wears “movie-quality” Chewbacca masks and conducts experiments in his “beat laboratory” (a drum room) with dreams of superstardom. (If Talladega flipped a bird to NASCAR, Brothers does so to the American Idol anyone’s-a-star mentality, with the finger in its pocket.)

After Brennan’s mom (Mary Steenburgen) and Dale’s dad (Richard Jenkins) meet and marry, the new stepbrothers clash with pouts and pranks after the families merge.

Here, the movie goes a little too long with profane shouting matches between the “boys.” It’s the little pouting-rant details of Ferrell and Reilly’s inner 12-year-olds that are better, such as perfectly clinched near-tears faces or fishing for whispered taunts at each other in a shared bedroom. (Credit costume designer Susan Matheson and production designer Clayton Hartley, for making this setting, and their wardrobe, a perfect petri dish of retro pop culture.)

Eventually bonding over a shared “favorite non-porno mag to masturbate to,” Brennan and Dale are soon enjoying karate in the garage and Steven Seagal movies in the den together. It’s when they’re forced to find work, and, eventually, fix a fast-fracturing family, that the film finds footing.

That’s because Step Brothers, in due time, doesn’t easily cast aside family members who have sour feelings as vile villains. It’s nowhere near an integral blend of comedy and drama as in Stuart Saves His Family, but it works. (One man calling another “Dragon” is a strangely moving gesture.)

Also, while not quite Anchorman’s expert ensemble, Steenburgen, Jenkins and Scott turn in solid work, as do Kathryn Hahn (as Derek’s wife, sexually awakened by Dale), the Mr. Incredible-framed Rob Riggle (as Derek’s crony) and Horatio Sanz as a beleaguered vocalist.

In all, it’s a movie that delivers greatly on the promise of its preview — no more, no less, just hilarious.