[from VAD, a permutation of ADV (i.e., ADVENT), used to avoid a particular admin's continual search-and-destroy sweeps for the game] A leisure-time activity of certain hackers involving the covert exploration of the ‘secret’ parts of large buildings — basements, roofs, freight elevators, maintenance crawlways, steam tunnels, and the like. A few go so far as to learn locksmithing in order to synthesize vadding keys. The verb is to vad (compare phreaking; see also hack, sense 9). This term dates from the late 1970s, before which such activity was simply called ‘hacking’; the older usage is still prevalent at MIT.
The most extreme and dangerous form of vadding is elevator rodeo, a.k.a. elevator surfing, a sport played by wrasslin' down a thousand-pound elevator car with a 3-foot piece of string, and then exploiting this mastery in various stimulating ways (such as elevator hopping, shaft exploration, rat-racing, and the ever-popular drop experiments). Kids, don't try this at home!