"Bad Santa" is one hilariously crass Christmas comedy -- and most certainly not for kids.

From the comical poor taste of running the title credit over a shot of a bitter, broken-down, booze-hound mall Santa (Billy Bob Thornton) upchucking in an alley behind a bar, to the antagonism-eroding friendship he strikes up with an overweight, none-too-bright, literally snot-faced kid (Brett Kelly) who follows him from a department store, this movie is fearlessly twisted and has only the slightest hint of traditional redemption.

But man, it is funny.

Haggard, unshaven Thornton (who couldn't be more perfectly cast) and sharp-tongued, 36-inch-tall Tony Cox ("Me, Myself and Irene") play Willie and Marcus, con artists who take seasonal jobs as Santa and elf so they can crack Christmas-fattened department store safes and live off the proceeds until the next year. Their operation is so low-budget that Cox's peach-colored pointy elf ears don't even match his black skin, and Willie so lacks the spirit of St. Nick that when a kid calls him on his fake beard, he sneers and says his real one fell out because "Santa slept with an unclean woman." And that's on one of his better days.

He dream is to "go to Miami, open a bar, stop drinking and marry a waitress." But in reality, Willie's more likely to wake up at 11:15 a.m., look at last night's flat beer with a cigarette butt floating in it, shrug, take a swig and call it breakfast.

But very, very slowly what's left of Willie's dormant humanity begins to emerge after he takes advantage of the one thick-headed kid who persistently believes he really is Santa, despite the drinking, the swearing, the haggard 170-pound frame and the tendency to fly into a paranoid fury when interrogated about life at the North Pole.

Seeing an opportunity to crash in style during a Christmas stint in Phoenix, Willie muscles his way into the suburban house where the kid lives with his near-invalid grandmother (Cloris Leachman) by saying "I'm gonna stay here for a while. Mrs. Santa caught me (with) her sister."

By day, the sloshed and hostile Willie continues to fall apart on the job, raising the ire of Marcus and suspicions of the milksop store manager (John Ritter) and the crooked head of security (Bernie Mac). By night, he eats the kid's advent calendar candy and doesn't even try to keep up the red-suited facade -- save wearing his cap when he brings home a bar maid with a Santa fetish (Lauren Graham) to have sex with her in the hot tub.

"Goodnight Santa," chirps the sweetly dopey kid that night. "Goodnight Mrs. Santa's sister."

The gullible boy's persistent faith does start to wear the creep down, if for no other reason than that he feels a tad guilty about taking such easy advantage. But if you're looking for a warm-fuzzy finale, look elsewhere because while director Terry Zwigoff ("Ghost World," "Crumb") has a heart, he isn't about to abandoned the film's endless potential for pitch-black humor. In "Bad Santa," a feel-good moment is when Willie shows he cares by beating the daylights out of a teenage skateboarder who picks on the kid.

The movie is not without its false moves -- the most significant of which is the miscasting of good-hearted cutie-pie Lauren Graham (the mom on "Gilmore Girls") as a woman who would be even remotely interested in a pathetic, acrimonious, wet-leather drunk -- no matter who he was dressed up as. What this role needed was an actress like Lili Taylor ("I Shot Andy Warhol," "High Fidelity") -- someone less twinkly who could sink her teeth into a bartender with the kind of basement-level self-esteem to believably latch on to this loser just to be latched on to anyone at all.

Please consider yourself warned: "Bad Santa" is not a movie that will earn peels of laugher from anyone who holds the joys of the season sacred. (Sorry, Mom.) But this cynical and bleak yet fringe-optimistic flick is the most side-splitting, balls-to-the-wall politically incorrect comedy since "There's Something About Mary."