Tracey Ullman is so perfectly attuned to John Waters' brand of lasciviously trashy comedy, it's a wonder that she hasn't worked before for the shamelessly silly provocateur.

In the uproarious "A Dirty Shame," the writer-director lets the caustic comedienne cut loose as Sylvia Stickles, a frigid, uptight working-class suburbanite who becomes an insatiable sex maniac by getting bonked on the noggin in a car accident.

After shocking her hitherto frustrated husband (played by singer Chris Isaak) with tongue-wiggling come-ons and liberating her trampy, triple-Z-cup stripper daughter (played with bimbonic irony by real-life A-cup Selma Blair) from the bedroom where she'd been padlocked away "for her own good," Sylvia joins other concussion-born libertines as a disciple of a self-proclaimed sexual evangelist (amusingly uncouth Johnny Knoxville). All of this helps set the stage for an absurdist battle against a band of spitefully self-righteous local prudes for the soul of their Baltimore neighborhood.

Mercilessly mocking those who fear sexual tolerance will unravel society, Waters delights in taking both puritanism and prurience to satirical extremes that earned "A Dirty Shame" an undeserved NC-17. (The R-rated "American Pie" movies are more graphic and at least as raunchy.)

He takes mirthful pot-shots at 12-step programs, peppers the picture with hilarious pseudo-subliminal messages (W-H-O-R-E and V-A-G-I-N-A flash on the screen, as do scenes from campy 1950s stag films) and builds his plot around the notion of a deviant minority pushing its "agenda" when its members could just as easily be "cured." You see, an epidemic of cracks to the cranium sends Sylvia (and pretty soon everyone else) swinging wildly back and forth between priggishness and debauchery.

As with all Waters' tongue-in-cheek (and other places) flicks, the story is somewhat undercooked, and humorous histrionics are the order of the day (notably from the tarted-up Ullman and Suzanne Shepherd as Sylvia's psychotically conservative mother). So it's not unexpected that "A Dirty Shame" has its moments of balderdash and ridiculously bad acting (notably from a neighborhood family of gay "bears"). But the laughs are relentless, and Waters' ultimate joke is on all of us because there's something here to fluster just about everyone -- be it an extreme prejudice, an eccentric sexual proclivity or an impudently exploited stereotype.