it would usually be used for someone who doesn't know anything PRACTICAL or USEFUL or DOWN-TO-EARTH "The professor is a brilliant physicist, but I swear he doesn't know one end of a pig from another. Yesterday I saw him filling his gas tank with diesel instead of gas."

I understand this, as an English native speaker, as an insult to one's intelligence. It is to say, that the person can tell no difference between the front end of a pig and the back end.