You’ve seen kids trapped in grown-up bodies in Big and 13 Going on 30. You’ve seen adults act impossibly childlike in Elf and…well, let’s face it, pretty much every Will Ferrell movie. But until now, the world had not seen the Saturday Night Live vet’s nut sack. So you can’t say that Step Brothers, Adam McKay’s comedy about two middle-aged, very hairy men who still live at home and throw hissy fits about the rules of their ’rents, doesn’t bring something new to the man-child table.

“I teabagged your fucking drum set,” Brennan (Ferrell) tells his new stepbrother, Dale (John C. Reilly), during a particularly heated dust-up, and damned if the Talladega Nights and Anchorman writer-director doesn’t actually zoom in on Brennan’s balls knocking against a snare drum. Even after Jason Segel fwapped full-frontal in the recent Forgetting Sarah Marshall—which, like Step Brothers, is a Judd Apatow production—Ferrell’s Big Reveal is shocking and inexplicably hilarious.

So is the rest of the movie. Co-written by Ferrell, Step Brothers immediately dirties its sitcom-ready path with humor that’s largely filthy and occasionally flat-out weird. “I’m watching the thing with the lady,” Brennan mutters to his mother, Nancy (Mary Steenburgen), who dares to try to interrupt him while he’s slumped on the couch gawping at Denise Austin. Soon, he’ll have to fight for control of the remote: Nancy has decided to marry Robert (The Visitor’s Richard Jenkins), a doctor she impetuously hooked up with at a conference. And Robert has a 40-year-old son living with him as well. “Hey Nancy, can you make me a grilled-cheese sandwich?” Dale asks his new stepmother the day their households merge. She’s game, but Robert stops her: “No, he’s testing you,” he says with all the seriousness of a father newly savvy to a toddler’s tricks. Another wrench in the new family’s life is Nancy’s other son, Derek (Adam Scott), a successful jackass with whom Richard becomes enthralled.

The genius of Step Brothers isn’t merely the reunion of Ferrell and Reilly, an inspired pairing in Talladega Nights that continues to show off a talent for absurd stream-of-consciousness riffing here. Better is the bizarre personality dissonance of the perpetually fighting characters. Brennan and Dale aren’t just losers who never found a good enough reason to leave the nest. They’re utter children, sobbing when the other’s barbs get too mean and clueless at showing affection. (Ferrell’s abstract near-hugs are priceless, and Reilly’s smooch with a married horndog is just about the funniest on-screen kiss yet.) But then the expletives fly: “This house is a prison!” “On Planet Bullshit!” “In the galaxy of This Sucks Camel Dicks!” they take turns yelling when Nancy and Robert demand they find jobs and move out. They’re familiar with terms such as “ball ’fro” and “mangina.” And the stepbrothers’ cursing also goes beyond the tee-hee usage associated with strutting tweens and can get as fuck-you nasty as a prison clash. One minute they’re babies, the next they’re men, and the back-and-forth keeps the script from becoming predictable. For every, say, 10 utterances of “fuck,” McKay throws out a word like “bugaboo.”

There are some brilliant sequences, including sleepwalking scenes in which Brennan and Dale eat, hide, and destroy things while muttering like zombies. With all the chances McKay takes here, though, it’s inevitable that Step Brothers doesn’t quite reach Anchorman heights: Misses include Ferrell’s timid rendition of “Something to Talk About”—it’s funnier when he simply tells Dale that he’s been called the “songbird of my generation”—and a nympho-goes-wild scene that feels ripped off from Wedding Crashers. Still, it’s a nice mini-comeback for Ferrell and Reilly, who needed to rid themselves of the stink of Semi-Pro and The Promotion, respectively, as well as a great turn by Jenkins, who must have regarded dialogue about not losing your inner T. Rex as a palate refresher after the serious themes of The Visitor. Steenburgen is a fine straight woman, too, and actually sums up the film’s appeal nicely: “This is offensive,” Nancy remarks while watching a terribly un-P.C. video the boys made. But she’s smiling.