Step Brothers is the greatest comedy of the past decade, both for everything it does and for the one key thing it doesn’t. Will Ferrell, John C. Reilly and director/co-writer Adam McKay’s 2008 collaboration is a preeminent work of man-child absurdity, a smorgasbord of nonsense involving grown men acting like adolescents who’ve been hit on the head a dozen times too many, and parents struggling to stomach their progeny’s willful inanity. It’s an exuberant salute to immaturity—and one that’s free from preachiness, upending expectations by refusing to partake in the sort of kill-joy moralizing that so many of its genre brethren feel obligated to indulge.

Part imbecilic-Peter-Pan fantasy, part empty-nester nightmare, and all riotous ridiculousness, it’s a masterpiece of morons being morons, as stupendously insane and amusing as it was when it debuted on July 25, 2008.

Presaging today’s epidemic of stay-at-home millennials by a good ten years, Step Brothers concerns two doofuses, 39-year-old Brennan Huff (Ferrell) and 40-year-old Dale Doback (Reilly), whose determination to stick by their single parents—Nancy Huff (Mary Steenburgen) and Dr. Robert Doback (Richard Jenkins)—is complicated by their respective mom and dad’s decision to marry. Thus a blended family is born, albeit in strife, because the only thing Brennan and Dale enjoy less than responsibility is the idea of having to share their lives with strangers. No sooner have these dimwits moved in together in the Doback house than they’re butting heads, engaging in weirdo staredowns across the dinner table and hurling insults at each other while lying in their parallel twin beds, with Dale telling Brennan that “You and your mom are hillbillies. This is a house of learned doctors,” and Brennan responding by informing his new sibling, “You better not go to sleep, ‘cause as soon as your eyes shut I’m going to punch you square in the face.”

The opening image of McKay’s film is that of a homemade plate of nachos—an apt sight for a story that’s simultaneously childish, delicious and uninterested in good-for-you nutritional value. From the get-go, Step Brothers revels in extremity. Dale is territorial about his drum set (which he demands Brennan not touch) to the point of lunacy. Their pranks on each other—Dale draws “I ♥ Crystal Meth” on Brennan’s polo shirt; Brennan decorates clueless Dale’s face with bloody-wound make-up—are above and beyond. Their kindred sleepwalking habits are impressively spasmodic. And their initial physical scuffle escalates to such a psychotic level that it eventually leads to Nancy suffering a blow and ends with Brennan claiming that he only went so far because “I honestly thought I was going to be raped for a second. He had the craziest look in his eyes. And at one point he said, ‘Let’s get it on.’”

Dale and Brennan’s hostility resembles that of most siblings, except taken to a bath-salts degree of madness. However, a run-in with Brennan’s insufferable younger brother Derek (Adam Scott)—who leads his clan in a cappella renditions of “Sweet Child O’ Mine” in the car, says things like “bro” and “shits and gigs,” and boasts about his $500K salary and the third-rate celebrities he hangs out with—soon bonds them together, courtesy of Dale punching Derek square in the smarmy face. Their newfound camaraderie is as loopy as their preceding enmity, expressed as it is through fruit-smashing karate sessions in the garage, construction of a makeshift bunk bed that goes very, very wrong, and, in one of the most delirious scenes in comedy history, their team-up on an “investor presentation” of their new entertainment company Prestige Worldwide, which is marked by the apex of music-video artistry.

As with Ferrell’s prior big-screen gems, Anchorman: The Legend of Ron Burgundy and Talladega Nights: The Ballad of Ricky Bobby—the latter also with Reilly—Step Brothers gets plenty of mileage out of wackadoo wordplay, be it Dale praising Brennan’s amazing singing talents (“Your voice is like a combination of Fergie and Jesus”), the duo’s waking-up-from-sleep mutterings (“I’ll kill you Leonard Nimoy”) or Brennan’s pet name for his psychiatrist Denise (“Dr. Angelface”), whom he’s convinced is his soulmate. Written by Ferrell, Reilly and McKay, the film’s back-and-forths feel as if they were born through improvisation, as each one tried to crack up the others. The effect is that one never knows when a randomly outrageous one-liner will materialize. Among other things, it may be the most quotable movie of the new century.

Still, all the bizarre put-downs and comebacks in the world would be nothing without Step Brothers’ formal acuity (McKay’s widescreen framing smartly accentuates his character’s push-pull dynamic) and narrative structural integrity, as the script captures just about every phase (marriage, maturity, discord, divorce, reconciliation) in a typical suburban family lifecycle. Nor would it thrive without the benefit of an across-the-board great cast. Reilly’s dim, scrunched-up scowl and Ferrell’s look of blank fury and disgust are perfectly harmonized—as is their gleeful stupidity when they join forces, especially during a job-interview sequence featuring tuxedos, farts, and the bonkers mispronunciation of the name “Pam.” They are, in short, an ideal pair of overgrown idiots, as comfortable relating over old Hustlers as they are trying to bury each other alive.

Steenburgen is a delight as chipper enabler Nancy; Scott is supremely unctuous as the Ice Ice Baby-flavored Derek; and Kathryn Hahn steals her every scene as Derek’s miserable wife Alice, who flings herself into Dale’s arms and crotch (after he cold-cocks Derek) and, in a men’s bathroom, pulls off a legendary feat (“Oh my god. You’re incredible!”). In bit parts, Seth Rogen and Rob Riggle are also on top of their game. Best of all is Jenkins, who proves himself an amazingly gifted funnyman as the beleaguered Robert. As his middle-aged progeny become ever-greater thorns in his side, and then outright destroy his dream of retiring and sailing around the world with Nancy, Robert’s frustration escalates into unbridled fury, culminating with a freak-out that, among its many virtues, features cinema’s all-time best use of the term “jagaloons.” How he hasn’t starred in twenty comedies since is a mystery.

What truly elevates Step Brothers above the stunted-adolescent fray, however, is its refusal to condemn—à la likeminded mid-aughts efforts such as Old School, Wedding Crashers and Knocked Up—the very dim-witted behavior in which it wallows. Instead of ending with its protagonists realizing the error of their juvenile ways, it paints such a development as borderline-tragic: the stereotypical late-second-act crisis of McKay’s film is Brennan and Dale’s success at getting jobs and learning to go to bed at a reasonable hour. As Robert says, upon seeing the two now conducting themselves as sensible baby aspirin-popping adults, “It just kills me to see you so crushed to normal.”

Step Brothers feels likewise, and thus takes a detour away from didacticism during its climax at the Catalina Wine Mixer, where Robert recounts his childhood habit of pretending to be a dinosaur, viewers are reminded that “’80s Billy Joel doo-wop sucks!”, and Brennan and Dale save the day by ditching their white-collar straightjackets to partner on a drums-and-vocals performance of Andrea Bocelli’s “Por Ti Volare.” Giving equal time to each of its stellar players, that joyous finale is punctuated by fantasies about centaurs and lumberjacks, and peaks with the most awkward love-hate hug ever committed to celluloid. As befitting Brennan’s operatic vocal performance, it’s a last high note in a saga filled with them.

Moreover, like the film itself, it’s an uproarious celebration of staying true to yourself—and, also, a reminder that (comedic) maturity is crazy overrated.