‘Dirty Grandpa.” The un-ironic title is like a spray-painted graffiti sign warning everyone to stay away from this sewage-swilling clogged cesspit masquerading as a movie. Woe betide anyone who catches this on an airplane, they may wind up using all the airsick bags in their row, because it really stinks.

The burning question that pops up two minutes into “Dirty Grandpa” has to be: Why doesn’t Hollywood treat its iconic stars with more respect? Two-time Oscar winner Robert De Niro, who has given us some of the most memorable characters in American cinema (Vito Corleone in “Godfather II,” Jake La Motta in “Raging Bull,” Al Capone in “Untouchables” to name just a few), stars as Dick (I’m not kidding) Kelly, a retired U.S. Army general who just lost his wife of 40 years.

In what feels like a mere seconds after the funeral, Dick is already masturbating to a porn movie, and when his grandson Jason (Zac Efron) walks in on him, he merely remarks that he’s almost done, without a trace of embarrassment. That’s not even a low point of the film, that’s just director Dan Mazer (“I Give it a Year”) flexing his arm muscles to throw s—- pies at the audience and, by implication, at De Niro who’s widely acknowledged as one of the greatest actors of all time.

But it’s not like Mazer held a gun to De Niro’s head, which, to me, would have been the only acceptable explanation for him taking this role. Bizarrely, De Niro seems to relish being Dick, even though it involves f-word staccato-hollering the an amazing five times in a single sentence (or was it eight?).

Apparently Dick hasn’t had sex in 15 years and now that he’s widowed, he’s determined to get some — in Daytona Beach, with the hormone-laden ardor of a virgin frat boy. And who does he take along for this endeavor? His very respectable grandson, who is about to get married.

A word about Jason: he’s everything that his grandfather isn’t — conventional, polite and a corporate lawyer. He’s so straightlaced and nice that he doesn’t have a problem letting his fiancee, Meredith (Julianne Hough), control not just their imminent wedding, but his whole life. If nothing else, Dick is at least right to lay it on the line for Jason: “If you marry her, you’ll wind up sleepwalking for the rest of your life.”

If only he stopped there. If only he were more like Al Pacino in “Scent of a Woman,” walking a young protege through life’s most important lessons. But no, Dick instead insists on hitting Daytona and its “chicks” and, surprisingly, Daytona girls want him right back. Many of them beckon to Dick like stars in a sick episode of a porn series for seniors. Especially eager is sexpot Lenore (Aubrey Plaza), who happens to be friends with Shadia (Zoey Deutsch), Jason’s college classmate, who just happens to be in town when the Dick-and-Jason duo roll in to don g-strings (seriously) and embed themselves in orgies.

Many hours after watching “Dirty Grandpa” — after you’ve recovered from the assault and are ready to use your brain again — it may strike you that this could be an attempt to make the ultimate statement about old age and the importance of living life to its fullest before it’s too late. De Niro certainly seems to think so — no one can say his Dick is pitiful or too bitter to know how to have a good time. By the time you get to entertain this thought, though, it’s likely you’ll be too numb to care.

According to the Wall Street Journal, the average life span of an American white male is 76½ years old — and Robert De Niro is now 73. Maybe the desperation he shows in this movie is not all acting. At one point, Dick makes it abundantly clear that old age is terrifying, and he’s ready to do anything to stave it off. Perhaps that’s what De Niro is also trying to do.

But even if this were the case and Mazer and Co. wanted to show that old age is not all bad, there just has to be an alternative to “Dirty Grandpa” to send that message. They should have gone to Diane Keaton for advice.