Yuletide tonic of choice — shaken, stirred and promptly spilled into the closest toilet.

A glum-faced, downtrodden Billy Bob Thornton courageously embodies the titular antihero, Willie, a burnt-out, alcoholic mall Santa. Vulgar, horny as hell and far from housebroken, Willie cusses at his adolescent clientele like a sailor on shore leave while always finding time before or during work to down a pint or two of whiskey, gin, bourbon … whatever’s lying around, really.

His professional behavior would be cause for alarm if the job wasn’t a front for him and his midget partner-in-crime, Marcus (Tony Cox), to rip off the malls. Every year, the criminal duo pose as Kris Kringle and a merry elf until Christmas Eve, during which Willie cracks the mall safe and

Marcus shoplifts presents for his wife. A smooth operation, except this ersatz Claus can’t keep his pants up in the Ladies Plus-Size department, nor can he pull them down before urinating on the job. Not to mention the alcohol: Willie could drink Nicolas Cage’s character from “Leaving Las Vegas” under the table in 10 minutes flat and still have time for a quick nightcap.

As bleak and depraved as the set-up sounds, it makes for a hilarious, mercilessly profane mainstream comedy. The “South Park” movie looks like

“Finding Nemo” by comparison.

Much of the humor lies in the details, the throwaway shots or little asides, e.g. a toddler mannequin’s head getting blown off during a shootout finale in the mall. I don’t know whether “Ghost World” director Terry Zwigoff — America’s auteur of misanthropy — or the screenwriters were responsible for such sight gags, although it’s telling that the Coen Brothers executive produced and are rumored to have tinkered with the screenplay.

Its holiday-spurning grotesqueries and bodily functions notwithstanding, “Bad Santa” reserves much of its unsparing wit and subversion for the

relationship between Willie and a rotund youngster (Brett Kelly) who offers his home and never-ending line of oblivious questioning.

The boy’s name is spoken only once and immediately forgotten — Willie and the closing credits stick with “Kid.” Cinematic father/son conventions are similarly discarded, as the one lesson Willie imparts to the boy is how to efficiently kick bullies in the testicles.

Also on hand for the mirth and naughtiness is Bernie Mac as the mall detective hot on the pair’s trail, and, in his final screen appearance, the

late John Ritter, playing a suspicious yet inept department manager. One must wonder how Ritter’s family feels about the film being dedicated in his loving memory.

Inexcusably ribald, contrived and all the more proud for it, “Bad Santa” is sure to be the perennial holiday favorite for curmudgeon families weary of

holiday pretense. In the meantime, potential viewers are advised to keep small children, the fainthearted and elderly relatives as far away as

possible.

Grade: A