In the prank TV show Jackass and a subsequent feature film, Johnny Knoxville donned latex to give octogenarians a bad name as politically incorrect grandpa Irving Zisman.
Director Dan Mazer and screenwriter John Phillips channel a similar vibe of old men behaving crudely in this raunchy cross-generational road trip that pairs raging bull Robert De Niro and wholesome High School Musical alumnus Zac Efron.
It’s a tantalizing juxtaposition - wizened, worldly experience and youthful exuberance - and Phillips’ expletive-laden script should mine a rich vein of humour by upending expectations about how these characters behave in polite society.
Alas, the drunken fraternity humour that runs rampant is wearisome and occasionally distasteful, including double standards in its treatment of homophobia.
De Niro visibly savours his feisty old coot’s potty-mouthed outbursts.
For his part, Efron gamely loses his shirt and his trousers, flashing his washboard stomach in a series of humiliations that include a mistaken case of exposure to a minor on a beach.
Seventy-something man of mystery Dick Kelly (De Niro) buries his wife and emotionally blackmails his grandson Jason (Efron) into driving him to their summer home in Florida.
‘It’s what she would have wanted,’ Dick assures Jason, who is a corporate lawyer in the same firm as his father (Dermot Mulroney) and is poised to walk down the aisle with his controlling fiancee (Julianne Hough).
Jason arrives at his grandfather’s home and walks in on the old timer in a state of gleeful undress, enjoying a pornographic film.
‘You caught me taking a number three,’ cackles Dick, without a flush of shame.
The two men hit the road and are soon diverted to Daytona Beach, where Jason has a chance to reignite romance with old flame Shadia (Zoey Deutch) while perpetually libidinous Dick pursues Shadia’s obliging friend, Lenore (Aubrey Plaza).
‘The greatest gift a grandson can give his grandfather is a hot college girl who wants to have unprotected sex with him before he dies,’ declares Dick, but standing in his way are loutish college dudes Cody (Jake Pickering) and Brah (Michael Hudson).
Dirty Grandpa drinks from the same filthy glassware as The Hangover and its bromantic brethren.
The two leads throw themselves into the fray with abandon, weathering numerous indignities including a topless dance-off in search of cheap laughs.
Amidst the filth, scriptwriter Phillips dispenses pat life lessons about taking charge of your destiny and respecting elders.
A running gag involving a drug dealer (Jason Mantzoukas) and two inept police officers (Mo Collins, Henry Zebrowski) runs out of puff while the sight of De Niro repeatedly shoving his thumb up his co-star’s bottom as a laddish prank gets a thumbs down on its first airing let alone the fourth or fifth.