No one ever said that English idioms have to make logical sense. I don’t think there are going to be too many circumstances where any old fellow could indeed be determined as the uncle of a monkey. I also don’t think that rainfall would ever get quite so torrential as to cause a number of our furry companions to fall out of the sky. You would hope that your thoughts are worth more than a penny. Even so, we must first start with logic.
And that’s why “hold down the fort” is technically incorrect. What you really mean is to “hold the fort.”
Let’s start with a simple definition. When you tell someone to “hold the fort” for you, you are telling them to watch over or take care of the place in your absence. Normally, this would describe a situation where you are typically present and responsible for the location, but you are leaving temporarily and need someone else to watch over it.
This is used in a figurative sense and it is analagous to a military situation.
In a more literal sense, you and your group could be defending your “fort” against invading marauders. The commanding officer for the army may have to leave for whatever reason and he may tell you to “hold the fort” until his return. You are “holding” the fort in that you are assuming responsibility for it, protecting it against the marauders. You are ensuring that the fort will still be “held” by your people when your commanding officer comes back.
It doesn’t make sense to “hold down the fort,” because the fort isn’t being held down. You are not pinning it to the ground. You are not preventing the fort from being blown away by a gust of wind or floating away like a helium balloon. You’re simply holding it for safe keeping.
So, just as you should never be caught saying that you “could care less” about a given topic or that a particular statement is true “for all intensive purposes,” you also shouldn’t tell someone to “hold down the fort” for you either. It just doesn’t make sense.