Okay: The line is characterizing someone as being so dumb that they could not pour piss out of a boot -- even if the instructions were written on the heel.
Just a tremendously clever line and if it does not have the impact today that it did when it was new it's because you've heard it already.
I have seen it attributed to LBJ and that would be something but it is more likely that it is an old Texas saying. One problem is, based on censorship laws, it could not have been in a movie and perhaps not in a book much before LBJ was born in 1908.
But it has a modern feel. Here's why: Prior to the advent of railroads, most goods were made locally -- towns were largely self-sufficient and it is doubtful anything you made or purchased had instructions written on it. Sears catalog might have had such things but that was late 19th century.
So I suspect we will not be able to find any indication that the saying was coined prior to the 20th century. If LBJ had made that up, would he not have made sure to let everyone know this? I sure would.
Now, I could believe that the first part was used -- just someone saying, why, that person couldn't pour piss out of a boot. The refinement could have been added maybe as late as the 1960s -- besides the argument about instructions being written on items, it just feels "edgy."