The term dry hopping originated centuries ago with British brewers and was used to refer to adding hops to the cask shortly before it was shipped off to the customer. In fact, 1/2-ounce hop plugs were specifically developed by British hop producers to be a convenient way to add whole hops to a keg or cask. Nowadays, dry hopping refers to any hop addition after the wort has been cooled. These additions can be done in the primary fermenter, in the secondary or by adding hops directly to a keg. I have even heard of one homebrewer attempting to add one or two hop petals to each bottle of a bottle-conditioned batch! (It didn’t work though.)
I would assume it's "Dry Hopping" because you are not adding any oils to the brew, just flavor and aroma. I did a quick google search and that's the best that I could find.