Old man boners. Did you laugh? Because if you didn't laugh, I can't think of any reason at all to bother with Jackass Presents: Bad Grandpa, a film for which "old man boners" is very largely the only joke it has up its sleeve. "Old man projectile shits", as well, but that's for just one scene, and like everything else in the movie, it disposes of that joke so quickly that it seems evident the filmmakers were ashamed of it.

Now, perhaps you did laugh at "old man boners", and I am not sorry if you did. The world would be less interesting if we all agreed on everything. And honesty compels me to acknowledge that as I left the theater where Bad Grandpa was playing, I distinctly heard a woman behind me gush, "That was so great". Honesty also compels me to mention that not once did that woman laugh during the movie, so perhaps when she called it "great", she was speaking to its depiction of the relationship between an old man and a young boy, finding in this one of the truest and most warming human stories of 2013. Again, the world would be less interesting if we all agreed on anything.

Bad Grandpa is, at any rate, an uncertain marriage of the hidden-camera extreme stunt comedy of the TV show Jackass and its three movie spin-offs (I have seen a couple bits from the show, nothing at all of the movies) with an improvised "embarrass the civilians" narrative generally modeled after Borat. Herein, octogenarian Irving Zisman (Jackass leading light Johnny Knoxville) is exuberant that his wife of more than forty years, Ellie (a model of Catherine Keener, whose actual scenes all ended up cut), has finally died, leaving him free to return to his days of tomcatting around. This unfortunately comes at the exact same moment that his daughter, Kimmie (Georgina Cates) is headed into prison on a drug charge, leaving Irving responsible for bringing his grandson, Billy (Jackson Nicoll) from Nebraska to North Carolina to be with the boy's wretched father Chuck (Greg Harris). The pair thus find themselves traveling across America, stopping off frequently for Irving to hit on the local women, and for some kind of violent mishap to occur to the undeniably game Knoxville.

There's an insurmountable taste issue here, which is that if you don't find the film's apparently inexhaustible well of genital-related jokes funny - I discovered how much I didn't by the end of the "elastic penis caught in a vending machine" routine that effectively opens the movie - there's not going to be much within Bad Grandpa's enthusiastically 92 minutes that will work on any level other than the sheer pleasure that comes from having moving images dance across your retinas. But it's not just taste, I suspect, that hobbles the film. It suffers mightily from repetition: most of the bits in the film that aren't jokes centered around the idea that an old man's privates are too ridiculous, gross, and funny for words are instead jokes centered around Zisman's insatiable lust, mostly realised in scenes of the old man hitting on women who, to their credit, are amused by the whole thing for the most part. The problem is that there are only so many ways for a 40-year-old in latex (that doesn't look very convincing, but nobody in the movie seems to doubt him) to describe sex acts in an old man voice, and Bad Grandpa has run through them all by the time the characters leave Nebraska. There's one joke repeated all but verbatim in the outtakes running during the credits - that is how unabashedly one-note this routine is.

The other huge problem I had is in the execution of the concept: the selling point of the movie is to watch normal, unknowing Americans respond incredulously to this vile old man being awful in front of, and to, his grandson. Which is fine, but the film is edited in such a way to absolutely minimise the reactions of all the real-life extras except in a couple of scenes (and those moments are, uniformly, the best - a funeral gone wrong, a preteen beauty pageant that dominated the ad campaign and is thus much less funny in the actual film than it should be), and that's back to the thing I mentioned at the start, that the movie seems ashamed of itself; every bit lasts exactly as long as it has to, not a second longer, and there is no lingering over the passers-by being confused, horrified, amused, or outraged, if there's any way to avoid it.

That leaves dicks, basically. Dicks and acting in a way to make people uncomfortable. But not too uncomfortable, and after hearing second-hand accounts of the things the Jackass folks were up to, I expected more depravity, honestly. The only reason I won't say that Bad Grandpa isn't as shocking as it thinks itself to be, is because it's not really very clear that it even thinks that it's shocking. Penis humor isn't much of a cinematic taboo anymore, and old people sex hasn't been taboo for years, just an easy way of scoring gross-out points. After a little while, Bad Grandpa can't come up with anything else to get a rise out of the audience than put the words "fuck" and "shit" in the mouth of a little kid, and if that's not the last refuge of the hopelessly lazy...

But again, taste issue. If you find exploding poo funny, then it's funny. If you find the thought of a geriatric man performing cunnilingus to be in and of itself funny, then it's funny. Bad Grandpa doesn't present these things in an inventive, creative, or surprising way, but it does present them with zest, passion, and sincerity. Knoxville cannot, at any rate, be accused of giving less than every inch of himself, and Nicoll has a perfect, unlearned ability to hold the camera with his open, eager face. The sheer volume is, eventually, enough to gin up something like amusement, anyway, even without finding the constituent parts worthwhile, though "the comedy wore me down to the point where I didn't mind it" is by no means what we might construe as an enthusiastic rave.