In English the word bag sounds a little bit like bad, dad and back.
A dad joke is normally a very bad joke that fathers tell topically but not exclusively on a long family holiday, when the family is stuck with the dad and have no plausible way to avoid the joke.
Dad jokes are not only told by fathers, of course, and a good dad joke, when timed well and delivered with sufficient conviction and charm, doesn’t necessarily have to be dad… I mean: bad 🙂 There are plenty of good dad jokes out there, I’m told.