Knoxville, buried under Oscar-nominated makeup is Irving Zisman, a Nebraska widower whose daughter is going to jail leaving her son Billy (Jackson Nicoll) alone. The boy's lowlife father in North Carolina agrees to take the kid (for the welfare money he'd get), so it's road trip time as the dirty old man and freakishly precocious kid find an escalating series of highjinks along the way.
Using a series of hidden camera bits and some stuff that's probably more scripted than they'd care to admit, the movie meanders from bit to bit with generally humorous to downright hilarious and occasionally terrifying effect. (Set pieces in a black strip club on ladies night and in a bar full of bikers veer close to "someone's gonna get killed here before anyone can explain it's a movie" terrain.)
Even though much of it is supposed to be "shocking" low-brow gross-out humor, the presence of Nicoll acts as a brake on what were probably even worse impulses the crew may've had, which may make it a tad safe (as far as a movie with prosthetic testicles danging down a foot can be safe) but still satisfies. The ace in the hole is Nicoll who has some man-on-the-street interactions where even if he was being fed lines via earwig, he still sells the bits hard. (I wondered if he was someone like Andy Milkonis who looked 14, but was 29; no the kid's actually that young.)
Someone "bad" has become a all-purpose prefix that indicates misbehaving main characters beginning with Bad Santa leading to Bad Teacher and some Bad Judge TV that's on know from what I've heard, so I suppose Bad Grandpa falls in line. But overall, it's not bad at all.
Score: 6/10. Catch it on cable.