1. To keep a fort from
flying away when the fort has such a low density that it tends to float off.
2. To take care of a place while someone who is usually there is gone, such as a store or one's home.
When unsure as to which definition should be applied, it can help to judge the person's
intelligence - morons usually mean #2. A less moronic term for #2 would be "to
hold the fort".