n. 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).
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!
See also hobbit (sense 2).