“Dirty Grandpa” stars Robert De Niro in the title role, a rambunctious senior citizen with sex on the brain and a vocabulary limited almost entirely to expletives. It’s ostensibly a comedy, though there’s nothing amusing about watching an Oscar winner’s talents going to waste. Crude in nearly every sense of the word, “Dirty Grandpa” is a nasty-humored, poorly made and desperately unfunny film.

We first meet Dick Kelly (De Niro) at his wife’s funeral, where he notices that his once-spirited grandson, Jason (Zac Efron), has grown into a joyless lawyer with the usual castrating fiancee (Julianne Hough as Meredith). Though the wedding is days away, Dick pressures Jason into driving him from Atlanta to Boca Raton. Ah, but that’s a ruse: Dick’s real destination is Daytona Beach, where he hopes to find a nubile Spring Breaker to sleep with — and, perhaps, convince Jason to loosen his own collar.

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Thin plots can result in great road-trip comedies. Just look at “The Sure Thing,” “Little Miss Sunshine” and “National Lampoon’s Vacation.” Those films, however, had intelligence, imagination and, most crucially, likable characters. “Dirty Grandpa” does not. Dick isn’t a person, just a checklist of bad behaviors like drinking, smoking and (especially) cursing. His sole function is to shock us, and he’ll do or say anything — racist jokes, gay slurs, exposing himself — to get a laugh. The film thinks he’s a lovable rogue. You’ll recognize him as a toxic personality.

Therein lies the main problem with “Dirty Grandpa.” Director Dan Mazer and writer John M. Phillips go out of their way to make every character repellent. The cute girl that catches Jason’s eye, Shadia (Zoey Deutch), is a self-righteous save-the-Earth type whose friends are snotty hippies. The film’s obligatory wacky guy (Jason Mantzoukas) peddles drugs to children. Dick’s willing coed, Lenore (Aubrey Plaza), is a creepy nymphomaniac with an AARP fetish. Even a tiny cameo from the usually endearing Danny Glover leaves an unpleasant taste.

The one actor who emerges from this disaster intact is Efron, who imbues Jason with more sensitivity and heart than the movie deserves. Otherwise, “Dirty Grandpa” soils everyone involved.