In climbing terms, one of the most god-damned boring and hideously injurious things you can do. Basically, find some small holds, hold on to them, and hang. Continue hanging until tired.

Problems with this? There are a few. One, its terribly boring. Insanely so. Considering most gym monkeys are under the age of 20, combining a boring exercise with low attention spans can lead to terrible unpredictable things. Like mixed climbing.

Secondly, if you love to tear your joints to pieces, this is the thing for you. Remember how when you went to lift weights you were told to flex your muscles throughout the entire movement? That wasn't just to make your muscles work harder. It's incredibly hard on connective tissue (read joints/tendons) to hold weight without your muscles helping out to stabilize/take some of the load. Dead hangs typically involve no muscle flex, just the slow death of your poor joints (usually elbows, as they take the brunt of your twisting carcass as you swing imperceptibly while hanging)

Moral of the story? Friends dont let friends do dead hangs. Climbing is about moving!