If I’d had access to the script for “Dirty Grandpa” before filming commenced, I seriously would have considered a crowd-sourcing effort to raise whatever funds necessary to pay Robert De Niro to NOT do this movie.

Mere words cannot do justice to how terrible this movie is from start to finish, but I’ll try anyway:

Awful, abhorrent, vile, offensive, hideous, unspeakable, atrocious, dreadful, appalling, revolting, odious, terrible, lousy, wretched, loathsome, repulsive, grim, repugnant, reprehensible, ugly and just really, really really bad.

Let’s go over a few of the running gags — gag being the operable word — in this unholy mess.

• Robert De Niro’s Dick Kelly, just one day removed from burying his wife of 40 years after her decade-long fight against cancer, thinks it’s the height of humor to shove his thumb into the rear end of his grandson Jason, played by Zac Efron.
• Dick is obsessed with having sex for the first time in 15 years. OBSESSED. It’s just the height of hilarity to hear Robert De Niro prattling on and on about how horny he is — not to mention the scene where Jason walks in on grandpa masturbating.
• On numerous occasions, various characters mistake Jason for a lesbian. It’s never clear why. The filmmakers apparently just thought it would be funny for random clueless individuals to look at Zac Efron and decide he was a lesbian.
• An obnoxious drug dealer named Pam — he’s a guy, but his name is short for Pamela, because “Dirty Grandpa” is obsessed with juvenile humor about gender — is a non-stop laugh machine, what with him tricking Jason into taking crystal meth, and using an ice cream truck as a perfect cover to sell drugs to children.
• Aubrey Plaza’s Lenore is a college student obsessed with having sex with horny ol’ grandpa. In scene after scene after scene, Lenore goes into graphic detail about what she’s going to do with this “dirty, dirty grandpa.”
• Through a typically tortured plot contrivance, Jason has to drive his fiance’s car, which is small and pink. This leads to Grandpa making joke after joke after joke comparing the car to a tampon and female genitals. Wacky!
It’s a toss-up as to which is more amateurish: the screenplay by John M. Phillips, which is peppered with obscenities and features such “edgy” humor as the appearance of a penis as a punch line (because that hasn’t been done to death over the last half-dozen years) and a cringe-inducing scene in which a “crew” of African-American tough guys get into a parking-lot brawl with Grandpa Dick, who just happens to be a former Special Forces operative who “trained insurgents behind enemy lines,” as he puts it.
Zac Efron’s Jason is an attorney engaged to Julianne Hough’s Meredith. We’re told Meredith is Jewish, the better to set up a tasteless and profoundly unfunny bit involving a swastika. Meredith is a pure cliché — the nagging, self-absorbed fiancé who whips out her phone at the funeral of Jason’s grandmother and presses Jason to make a decision about which color tie he’ll wear at the wedding. Your child’s stick-figure drawings have more depth and a richer backstory than this Meredith person.
Zoey Deutch plays Shadia, Jason’s former photography class partner, whom he conveniently bumps into as he’s driving Gramps to Florida. Turns out Shadia and the aforementioned Lenore and their mincing gay friend Bradley (Jeffrey Bower-Chapman), who looks to be about 35, are all heading to Daytona Beach, Florida, for spring break.
OK but wait. Jason and Shadia were classmates at one point. Now Jason is a junior partner in a law firm, but Shadia is still partying at spring break? Is she on the 10-year plan? What is happening!
Danny Glover surfaces for one of the most humiliating cameos in recent memory. Adam Pally bombs as Jason’s cousin, a horrible and wildly unfunny jerk. Henry Zabowski and Mo Collins further deaden the proceedings as inept, corrupt cops who are beyond stupid but never amusing.
Robert De Niro is of course one of the great dramatic actors we’ve ever known. And we learned a long, long time ago he’s capable of being funny as hell in the right vehicle.
This dreck, though, just makes you hang your head for the great De Niro, and hope someone somewhere is writing a role for him worthy of his legacy.
If “Dirty Grandpa” isn’t the worst movie of 2016, I have some serious cinematic torture in my near future.

Zero stars

Lionsgate presents a film directed by Dan Mazer and written by John M. Phillips. Running time: 97 minutes. Rated R (for crude sexual content throughout, graphic nudity, and for language and drug use). Opens Friday at local theaters.